theorem Th27:
  dom g = cod f implies g(*)f = (@g)*(@f)
proof
  assume
A1: dom g = cod f;
  then dom(@g) = cod f by Def9;
  then
A2: dom(@g) = cod(@f) by Def10;
  thus g(*)f = (the Comp of Ens(V)).(@g,f) by A1,CAT_1:16
    .= (@g)*(@f) by A2,Def11;
end;
