theorem
  C = x-term or
  ex o,p st p is x-context_including & C = o-term p
  proof
    per cases by Th16;
    suppose ex s,x st C = x-term;
      then consider s0 being SortSymbol of S, y being Element of X.s0 such that
A1:   C = y-term;
      s0 = s & x = y by A1,Th27;
      hence thesis by A1;
    end;
    suppose ex o,p st C = o-term p;
      then consider o,p such that
A2:   C = o-term p;
      thus thesis by A2,Th53;
    end;
  end;
