theorem
  for f being Function holds Z|`<:f,X,Y:> = <:f,X,Z /\ Y:>
proof
  let f be Function;
  thus Z|`<:f,X,Y:> = Z|`(Y|`(f|X)) by RELAT_1:109
    .= (Z /\ Y)|`(f|X) by RELAT_1:96
    .= <:f,X,Z /\ Y:> by RELAT_1:109;
end;
