reconsider g = f . (x1,x2) as Element of Funcs (A,D) ;
A1: [x1,x2] in [:D1,D2:] ;
( dom f = [:D1,D2:] & dom g = A ) by FUNCT_2:def 1;
then f .. (x1,x2,x) = g . x by A1, FUNCT_5:38;
hence f .. (x1,x2,x) is Element of D ; :: thesis: verum