elem is the list membership predicate, usually written in infix
form, e.g., x `elem` xs. For the result to be False,
the list must be finite; True, however, results from an element
equal to x found at a finite index of a finite or infinite
The has function returns True if the element appears in
the list; it is equivalent to member except the order of the
arguments is reversed, making it a function from an ordered list to
its characteristic function.