exclude some words from singular transformation

utils.py

@@ -223,6 +223,9 @@ MATH_WORDS = [
     "distribution",
     "distributive",
     "divide",
+    "divided",
+    "divides",
+    "dividing",
     "dividend",
     "divisibility test",
     "divisible by",
@@ -442,7 +445,12 @@ MATH_WORDS = [
     "modal",
     "mode",
     "multipl",
+    "multiply",
+    "multiplied",
+    "multiplies",
     "multiple",
+    "multiplication",
+    "multiplying",
     "multiplication counting principle",
     "multiplication diagram",
     "multiplication fact",
@@ -805,6 +813,10 @@ MATH_WORDS = [
     "zero",
 ]
 
+PLURAL_TO_SINGULAR_EXCLUSIONS = [
+    "plus",
+    "minus",
+]
 
 p = inflect.engine()
 
@@ -815,6 +827,8 @@ def singular_to_plural(word):
 
 def plural_to_singular(word):
     """Convert plural word to singular using inflect."""
+    if word in PLURAL_TO_SINGULAR_EXCLUSIONS:
+        return word
     return p.singular_noun(word) or word 
 
 plural_MATH_WORDS = [singular_to_plural(word) for word in MATH_WORDS]