
theorem Th18:
for A,X be set, B be Subset of X holds dom(chi(A,X)|B) = B
proof
   let A,X be set, B be Subset of X;
   dom(chi(A,X)|B) = dom chi(A,X) /\ B by RELAT_1:61
    .= X /\ B by FUNCT_2:def 1;
   hence thesis by XBOOLE_1:28;
end;
