Further Examples: FreeQ
Here is a feature of FreeQ that is absent from MemberQ: FreeQ recognizes that the sum a+c is a subexpression of the larger sum.
Here we see how the level argument to FreeQ affects its result. When no level is specified, matches at any level are reported.
The default level specification of FreeQ is . (MemberQ's is .)
The default for the Heads option of FreeQ is True.
This gives True since except for the head, this expression is free of ww.
See also the Further Examples for the Heads option.