reconsider g = f . x1,x2 as Element of Funcs A,D ;
( dom f = [:D1,D2:] & dom g = A & [x1,x2] in [:D1,D2:] ) by FUNCT_2:def 1;
then f .. x1,x2,x = g . x by FUNCT_5:45;
hence f .. x1,x2,x is Element of D ; :: thesis: verum