Removed unused import.

src/rules/numerics.py

 from itertools import combinations
 
 from .utils import greatest_common_divisor
-from ..node import ExpressionLeaf as Leaf, Scope, negate, OP_ADD, OP_DIV, \
-        OP_MUL, OP_POW
+from ..node import ExpressionLeaf as Leaf, Scope, OP_ADD, OP_DIV, OP_MUL, \
+        OP_POW
 from ..possibilities import Possibility as P, MESSAGES
 from ..translate import _