let x be set ; :: according to VALUED_2:def 5 :: thesis: ( x in Funcs X,Y implies x is rational-valued Function )
assume x in Funcs X,Y ; :: thesis: x is rational-valued Function
then consider f being Function such that
A1: x = f and
A2: ( dom f = X & rng f c= Y ) by FUNCT_2:def 2;
reconsider f = f as PartFunc of X,Y by A2, RELSET_1:11;
f is set ;
hence x is rational-valued Function by A1; :: thesis: verum