reserve a,b,c,v,v1,x,y for object;
reserve V,A for set;
reserve d for TypeSCNominativeData of V,A;

theorem Th2:
  for a,b,c,d,e,f being object holds dom {[a,b],[c,d],[e,f]} = {a,c,e}
  proof
    let a,b,c,d,e,f be object;
A1: dom {[a,b],[c,d]} = {a,c} by RELAT_1:10;
A2: dom {[e,f]} = {e} by RELAT_1:9;
    {[a,b],[c,d],[e,f]} = {[a,b],[c,d]} \/ {[e,f]} by ENUMSET1:3;
    hence dom {[a,b],[c,d],[e,f]} = dom {[a,b],[c,d]} \/ dom {[e,f]}
    by XTUPLE_0:23
    .= {a,c,e} by A1,A2,ENUMSET1:3;
  end;
