Added simplify_by_equality

Added a simplification function to simplify expressions using a given equality a = b + c, by replacing occurences of e.g. b + c by a, or a - b by c.

pystencils/sympyextensions.py

@@ -469,9 +469,12 @@ def simplify_by_equality(expr, a, b, c):
     if not isinstance(a, sp.Symbol) or not isinstance(b, sp.Symbol):
         raise ValueError("a and b must be symbols.")
 
+    c = sp.sympify(c)
+
+    if not (isinstance(c, sp.Symbol) or is_constant(c)):
+        raise ValueError("c must be either a symbol or a constant!")
+
     expr = sp.sympify(expr)
-    if not {a, b, c} <= expr.atoms(sp.Symbol):
-        return expr
 
     expr_expanded = sp.expand(expr)
     a_coeff = expr_expanded.coeff(a, 1)
@@ -481,11 +484,9 @@ def simplify_by_equality(expr, a, b, c):
     if isinstance(c, sp.Symbol):
         c_coeff = expr_expanded.coeff(c, 1)
         rest = expr_expanded - (c * c_coeff).expand()
-    elif is_constant(c):
+    else:
         c_coeff = expr_expanded / c
         rest = 0
-    else:
-        raise ValueError("c must be either a symbol or a constant!")
 
     a_summands = summands(a_coeff)
     b_summands = summands(b_coeff)