\DOC mem \TYPE {mem : ''a -> ''a list -> bool} \SYNOPSIS

\DOC mem
\TYPE {mem : ''a -> ''a list -> bool}
Tests whether a list contains a certain member.
list, set, eqtype.
An invocation {mem x [x1,...,xn]} returns {true} if some {xi}
in the list is equal to {x}. Otherwise it returns {false}.
Never fails.
Note that the type of the members of the list must be an SML equality
type. If set operations on a non-equality type are desired, use the `op_'
variants, which take an equality predicate as an extra argument.
High performance finite set operations may be found in the ML Standard
Basis Library.
Lib.op_mem, Lib.insert, Lib.find, Lib.tryfind, Lib.exists, Lib.all,
Lib.assoc, Lib.rev_assoc.