theorem Th123:
  for t being set holds t in NonTerminals DTConMSA MSVars C iff
  t = [ast C, the carrier of C] or
  t = [non_op C, the carrier of C] or
  ex c being constructor OperSymbol of C st t = [c, the carrier of C]
proof
  let t be set;
  set X = MSVars C;
A1: NonTerminals DTConMSA X = [:the carrier' of C,{the carrier of C}:]
  by Th120;
  hereby
    assume t in NonTerminals DTConMSA MSVars C;
    then consider a,b being object such that
A2: a in the carrier' of C and
A3: b in {the carrier of C} and
A4: t = [a,b] by A1,ZFMISC_1:def 2;
    reconsider a as OperSymbol of C by A2;
A5: b = the carrier of C by A3,TARSKI:def 1;
    a is constructor or a is not constructor;
    hence t = [ast C, the carrier of C] or t = [non_op C, the carrier of C] or
    ex c being constructor OperSymbol of C st t = [c, the carrier of C]
    by A4,A5;
  end;
  the carrier of C in {the carrier of C} by TARSKI:def 1;
  hence thesis by A1,ZFMISC_1:87;
end;
