diff --git a/SYMBOLS_MANIFEST.txt b/SYMBOLS_MANIFEST.txt
index 5815b2485..6b3bd147b 100644
--- a/SYMBOLS_MANIFEST.txt
+++ b/SYMBOLS_MANIFEST.txt
@@ -131,6 +131,7 @@ System`Arrow
 System`Arrow3DBox
 System`ArrowBox
 System`Arrowheads
+System`AssociateTo
 System`Association
 System`AssociationQ
 System`Assuming
@@ -634,6 +635,7 @@ System`KelvinBer
 System`KelvinKei
 System`KelvinKer
 System`Key
+System`KeyDropFrom
 System`KeyExistsQ
 System`Keys
 System`Khinchin
diff --git a/mathics/builtin/associations/modifying.py b/mathics/builtin/associations/modifying.py
new file mode 100644
index 000000000..2f09158f6
--- /dev/null
+++ b/mathics/builtin/associations/modifying.py
@@ -0,0 +1,90 @@
+"""
+Modifying Associations
+"""
+
+from typing import Optional
+
+from mathics.core.atoms.associations import Association
+from mathics.core.attributes import A_HOLD_FIRST, A_PROTECTED
+from mathics.core.builtin import Builtin
+from mathics.core.element import BaseElement
+from mathics.core.evaluation import Evaluation
+from mathics.eval.associations.modifying import eval_AssociateTo, eval_KeyDropFrom
+
+
+class AssociateTo(Builtin):
+    """
+    <url>
+      :WMA link:
+      https://reference.wolfram.com/language/ref/AssociateTo.html</url>
+
+    <dl>
+      <dt>'AssociateTo'[$a$, $key$ -> $val$]
+      <dd>Adds key-value pair $key$ -> $val$ to $a$.
+
+    </dl>
+
+    Create an association
+    >> assoc = Association[{a -> 1, b -> 2, c -> 3}]
+     = <|a ⇾ 1, b ⇾ 2, c ⇾ 3|>
+
+    Change the value associated with key $a$:
+    >> AssociateTo[assoc, a -> 10]
+     = <|a ⇾ 10, b ⇾ 2, c ⇾ 3|>
+
+    Add key $d$ rather than change it since it is not already in $a$:
+    >> AssociateTo[assoc, d->4]
+     = <|a ⇾ 10, b ⇾ 2, c ⇾ 3, d ⇾ 4|>
+
+    Change and add keys using several keys-value pairs:
+    >> AssociateTo[assoc, {d -> 5, e-> f}]
+     = <|a ⇾ 10, b ⇾ 2, c ⇾ 3, d ⇾ 5, e ⇾ f|>
+    """
+
+    attributes = A_HOLD_FIRST | A_PROTECTED
+    eval_error = Builtin.generic_argument_error
+    expected_args = 2
+
+    summary_text = "update an Association"
+
+    def eval(
+        self, a: BaseElement, expr: BaseElement, evaluation: Evaluation
+    ) -> Optional[Association]:
+        """AssociateTo[a_, expr_]"""
+
+        return eval_AssociateTo(a, expr, evaluation)
+
+
+class KeyDropFrom(Builtin):
+    """
+    <url>
+      :WMA link:
+      https://reference.wolfram.com/language/ref/KeyDropFrom.html</url>
+
+    <dl>
+      <dt>'KeyDropFrom'[$a$, $key$]
+      <dd>Removes key pair from association $a$.
+
+    </dl>
+
+    >> assoc = <|a ⇾ 1, b ⇾ 2|>
+     = <|a ⇾ 1, b ⇾ 2|>
+    >> KeyDropFrom[assoc, a]
+     = <|b ⇾ 2|>
+
+    >> KeyDropFrom[assoc, {b, d}]
+     = <||>
+    """
+
+    attributes = A_HOLD_FIRST | A_PROTECTED
+    eval_error = Builtin.generic_argument_error
+    expected_args = 2
+
+    summary_text = "remove a key from an Association"
+
+    def eval(
+        self, a: BaseElement, key: BaseElement, evaluation: Evaluation
+    ) -> Optional[Association]:
+        """KeyDropFrom[a_, key_]"""
+
+        return eval_KeyDropFrom(a, key, evaluation)
diff --git a/mathics/builtin/messages.py b/mathics/builtin/messages.py
index 4a729ef02..1ec62af48 100644
--- a/mathics/builtin/messages.py
+++ b/mathics/builtin/messages.py
@@ -215,6 +215,8 @@ class General(Builtin):
         "intp": "Positive integer expected.",
         "intnn": "Non-negative integer expected.",
         "intnm": "Non-negative machine-sized integer expected at position `1` in `2`.",
+        "invak": "The argument `1` is not a valid Association.",
+        "invlb": "The argument `1` is not a list, Rule or Association.",
         "invrl": "The argument `1` is not a valid Association or a list of rules.",
         "iterb": "Iterator does not have appropriate bounds.",
         "ivar": "`1` is not a valid variable.",
diff --git a/mathics/core/atoms/associations.py b/mathics/core/atoms/associations.py
index 64d90b69c..3523e9ccc 100644
--- a/mathics/core/atoms/associations.py
+++ b/mathics/core/atoms/associations.py
@@ -1,6 +1,6 @@
 """
-Mathics3 implementation of an Association atom
-6"""
+Mathics3 implementation of an Association atom.
+"""
 
 from typing import Any, Iterable, Optional
 
@@ -19,7 +19,7 @@ class Association(Atom, BoxElementMixin):
     """An Association is an Atom collection that maps keys to values,
     similar to a Python dictionary.
 
-    Each Key-Value mappings of an Association is called a Rule; but
+    Each key-value mapping of an Association is called a Rule; but
     this kind of Rule is distinct from (or a degenerate form of) the
     pattern-matching RewriteRules found in DelayedRule and Set
     builtins.
@@ -42,10 +42,9 @@ def __init__(self, elements: Iterable, expr: Optional[Expression] = None):
                     raise TypeError(f"Association keys must be Rules, got {rule_expr}")
                 self.collection[rule_expr.elements[0]] = rule_expr.elements[1]
 
-        self.update_for_change()
         return
 
-    # Add some dictionary like methods so that we can treat an Association object
+    # Add some dictionary-like methods so that we can treat an Association object
     # as we would a dictionary.
 
     def __delitem__(self, key: BaseElement) -> None:
@@ -60,10 +59,8 @@ def __delitem__(self, key: BaseElement) -> None:
         Side effects:
             Updates self.collection
         """
-        if key not in self.collection:
-            raise KeyError(key)
-
         del self.collection[key]
+        self._expr = None
 
     def __eq__(self, other: Any) -> bool:
         """Check equality with another Association."""
@@ -74,8 +71,8 @@ def __eq__(self, other: Any) -> bool:
             return False
 
         # "other" is an Association that is not literal like us,
-        # and has the same number items in its collection.
-        # Here, we have compare key-value pairs
+        # and has the same number of items in its collection.
+        # Here, we have to compare key-value pairs.
         return self.collection == other.collection
 
         # If for some reason the above does not work:
@@ -105,7 +102,15 @@ def __getitem__(self, key: Any) -> Any:
             return self.collection[key]
         raise KeyError(key)
 
+    # FIXME: We probably shouldn't have this.
+    # Find out what needs it and adjust that.
     def __hash__(self) -> int:
+        hash_elements = []
+        for key, value in self.collection.items():
+            # Update hash component
+            hash_elements.append((hash(key), hash(value)))
+
+        self._hash = hash(("Association", tuple(hash_elements)))
         return self._hash
 
     def __setitem__(self, key: BaseElement, value: BaseElement) -> None:
@@ -194,11 +199,28 @@ def head(self) -> Symbol:
         return SymbolRule
 
     def items(self):
-        """Return the values of an the association.
+        """Return the values of the association.
         Behaves like dict.items().
         """
         return self.collection.items()
 
+    def pop(self, key: BaseElement) -> BaseElement:
+        """pops a key-value pair from the association.
+
+        Args:
+            key: The key to remove.
+
+        Raises:
+            KeyError: If the key is not found in the association.
+
+        Side effects:
+            Updates self.collection
+        """
+        popped = self.collection.pop(key, None)
+        if popped:
+            self._expr = None
+        return popped
+
     def sameQ(self, other: Any) -> bool:
         """
         Mathics3 SameQ comparison.
@@ -221,34 +243,27 @@ def sameQ(self, other: Any) -> bool:
         return True
 
     def to_python(self, *args, **kwargs) -> Optional[dict]:
-        # FIXME
-        return None
+        result = {}
+        for key, value in self.collection.items():
+            if hasattr(key, "to_python") and hasattr(value, "to_python"):
+                result[key.to_python()] = value.to_python()
+            else:
+                return None
+        return result
 
     def to_sympy(self, **kwargs):
         return None
 
     def values(self):
-        """Return the values of an the association.
+        """Return the values of the association.
         Behaves like dict.values().
         """
         return self.collection.values()
 
     def update(self, e: Iterable):
-        """Return the values of an the association.
+        """Return the values of the association.
         Behaves like dict.update() except we return the update object
         value
         """
         self.collection.update(e)
-        self.update_for_change()
         self._expr = None
-
-    def update_for_change(self):
-        """
-        Things we have to do when the Association is changed.
-        """
-        hash_elements = []
-        for key, value in self.collection.items():
-            # Update hash component
-            hash_elements.append((hash(key), hash(value)))
-
-        self._hash = hash(("Association", tuple(hash_elements)))
diff --git a/mathics/eval/associations/__init__.py b/mathics/eval/associations/__init__.py
index be91ec16e..6494054e0 100644
--- a/mathics/eval/associations/__init__.py
+++ b/mathics/eval/associations/__init__.py
@@ -1,4 +1,4 @@
 """
 Evaluation routines and associated code for Built-in functions found under module
-mathics.builtins.assications.
+mathics.builtins.associations.
 """
diff --git a/mathics/eval/associations/modifying.py b/mathics/eval/associations/modifying.py
new file mode 100644
index 000000000..52b701804
--- /dev/null
+++ b/mathics/eval/associations/modifying.py
@@ -0,0 +1,81 @@
+"""
+Evaluation routines for builtins in mathics.builtin.associations.modifying
+"""
+
+from typing import Optional
+
+from mathics.core.atoms.associations import Association
+from mathics.core.attributes import A_PROTECTED
+from mathics.core.element import BaseElement
+from mathics.core.evaluation import Evaluation
+from mathics.core.expression import Expression
+from mathics.core.list import ListExpression
+from mathics.core.rules import is_rule
+from mathics.core.symbols import Symbol
+from mathics.core.systemsymbols import SymbolAssociation
+
+
+def eval_AssociateTo(
+    a: BaseElement, expr: BaseElement, evaluation: Evaluation
+) -> Optional[Association]:
+    """AssociateTo[a_, expr_]"""
+
+    # FIXME: we should test for Readable? And Writable?
+    attributes = 0
+    if isinstance(a, Symbol):
+        assoc_value = a.evaluate(evaluation)
+        attributes = evaluation.definitions.get_attributes(a.name)
+    else:
+        assoc_value = expr
+
+    if not isinstance(assoc_value, Association):
+        evaluation.message("AssociateTo", "invak", assoc_value)
+        return
+
+    if is_rule(expr):
+        changed = Association([expr])
+    elif isinstance(expr, ListExpression):
+        changed = Association(expr.elements)
+    else:
+        changed = expr
+
+    if (attributes & A_PROTECTED) or isinstance(a, Expression):
+        evaluation.message("Set", "write", SymbolAssociation, a)
+        return
+
+    if not isinstance(changed, Association):
+        evaluation.message("AssociateTo", "invlb", expr)
+        return
+
+    assoc_value.update(changed.collection)
+    return assoc_value
+
+
+def eval_KeyDropFrom(
+    a: BaseElement, key: BaseElement, evaluation: Evaluation
+) -> Optional[Association]:
+    """KeyDropFrom[a_, key_]"""
+
+    # FIXME: we should test for Readable? And Writable?
+    attributes = 0
+    if isinstance(a, Symbol):
+        assoc_value = a.evaluate(evaluation)
+        attributes = evaluation.definitions.get_attributes(a.name)
+    else:
+        assoc_value = a
+
+    if (attributes & A_PROTECTED) or isinstance(a, Expression):
+        evaluation.message("Set", "write", SymbolAssociation, a)
+        return
+
+    if not isinstance(assoc_value, Association):
+        # If it's not an association, we might just return it or handle errors.
+        return a
+
+    if isinstance(key, ListExpression):
+        keys = key.elements
+    else:
+        keys = [key]
+    for key in keys:
+        assoc_value.pop(key)
+    return assoc_value
diff --git a/mathics/repl.py b/mathics/repl.py
index a09748a8e..6afb0cc00 100644
--- a/mathics/repl.py
+++ b/mathics/repl.py
@@ -123,7 +123,7 @@ def __init__(
         except ImportError:
             pass
 
-        # Try importing colorama to escape ansi sequences for cross platform
+        # Try importing colorama to escape ANSI sequences for cross-platform
         # colors
         try:
             from colorama import init as colorama_init
@@ -212,7 +212,7 @@ def get_out_prompt(self, form=None) -> str:
         """
         Return a prompt string to be shown before showing output.
         """
-        line_number = self.last_line_number
+        line_number = self.last_line_number - 1
         if form:
             return "{3}{0}[{4}{1}{5}]//{2}= {6}".format(
                 self.out_prefix, line_number, form, *self.outcolors
diff --git a/test/builtin/associations/test_modifying.py b/test/builtin/associations/test_modifying.py
new file mode 100644
index 000000000..99a97a777
--- /dev/null
+++ b/test/builtin/associations/test_modifying.py
@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+"""
+Unit tests for mathics.builtins.associations.modifying
+"""
+from test.helper import check_evaluation
+
+import pytest
+
+
+@pytest.mark.parametrize(
+    ("str_expr", "expected_messages", "str_expected", "assert_message"),
+    [
+        (
+            "AssociateTo[1, 1⇾2]",
+            ["The argument 1 ⇾ 2 is not a valid Association."],
+            "AssociateTo[1, 1⇾2]",
+            "AssociateTo first parameter should be an association not an Integer.",
+        ),
+        (
+            "Clear[x]; AssociateTo[x, 1⇾2]",
+            ["The argument x is not a valid Association."],
+            "AssociateTo[x, 1⇾2]",
+            "AssociateTo first parameter should have a value.",
+        ),
+        (
+            "assoc = <|a⇾1|>; AssociateTo[assoc, 1]",
+            ["The argument 1 is not a list, Rule or Association."],
+            "AssociateTo[assoc, 1]",
+            "AssociateTo first parameter wrong type.",
+        ),
+    ],
+)
+def test_AssociateTo(str_expr, expected_messages, str_expected, assert_message):
+    check_evaluation(
+        str_expr,
+        str_expected,
+        failure_message=assert_message,
+        expected_messages=expected_messages,
+    )