The selectfun command searches an expression e for the function with operator name f, or, if f is a list or set of names, then selectfun searches e for any one of these functions.

It returns a set consisting of all the matching subexpressions g of e such that either op(0,g)=f or member(op(0,g),f) according to whether f is a name or is a list or set of names.

If the optional third argument x is specified, the selectfun command requires the subexpressions g be function(s) f of x (or function(s) f of a member of x if x is a set or list).