don't convert plain negation into fused-multiple add

src/pystencils/fast_approximation.py

@@ -220,13 +220,14 @@ def insert_fma(term, operators):
         # Find Mul with three factors, one of them -1, which can be fused
         elif expr.func == sp.Mul and -1 in expr.args:
             expr = flatten(expr)
-            factors = list(expr.args)
-            factors.remove(-1)
-            factors = [visit(f) for f in factors]
-            if '-*+' in operators:
-                return fnmadd(factors[0], sp.Mul(*factors[1:]), 0)
-            elif '-*-' in operators:
-                return fnmsub(factors[0], sp.Mul(*factors[1:]), 0)
+            if len(expr.args) > 2:
+                factors = list(expr.args)
+                factors.remove(-1)
+                factors = [visit(f) for f in factors]
+                if '-*+' in operators:
+                    return fnmadd(factors[0], sp.Mul(*factors[1:]), 0)
+                elif '-*-' in operators:
+                    return fnmsub(factors[0], sp.Mul(*factors[1:]), 0)
         new_args = [visit(a) for a in expr.args]
         return expr.func(*new_args) if new_args else expr