An expression expr is of type algfun (algebraic function) if it is an expression in the variable(s) V over the domain K extended by (polynomial) RootOfs.

The domain specification K must be a type name, such as rational or anything.  If K is omitted, then it defaults to type constant.

The optional argument V is an indeterminate or a list or set of indeterminates.  If V is not specified, then all the indeterminates of expr, which are names, are used.  That is, expr must be an algebraic function in all of its variables.