Hi, While unit-testing pyginac, I found the following inconsistency: 1) define a = symbol("a") b = symbol("b") c = constant("c") 2) observe content(2,a) -> 0 (ok) content(2*a + 4*c,a) -> 2 (ok) content(a*c,a) -> exception 'invalid expression in unit()' ?! content(a*b,a) -> b (ok) 3) similar behaviour can be observed also for other functions like unit,.. The question is: Is it possible to unify the behaviour of the relevant functions? That is, to extend them to accept other objects in addition to symbol, numeric, add, etc. Or, to raise an exception whenever there are alien object in expressions. (Though, constants should not be alien when comparing them with symbols.) My argument to consider this inconsistency as a problem is that this behaviour is confusing and could lead to difficult-to-find bugs. Thanks, Pearu