type
  TOddNumber = (one, three, five);
  TEvenNumber = (two, four, six);

var
  x: TOddNumber;
  y: TEvenNumber;

  var IncOddNumber(a: TOddNumber): TOddNumber;
  begin
    result :=TOddNumber(a + 1);
  end;

  var IncEvenNumber(a: TEvenNumber): TEvenNumber;
  begin
    result := TEvenNumber(a + 1);
  end;

  var IncAnyNumber(a: variant): variant;
  begin
    if a is TEvenNumber then 
      result := TEvenNumber(a + 1)
    else
      result := TOddNumber(a + 1);
  end;

begin
  x := one;
  y := two;

  x := IncOddNumber(x); //three
  y := IncEvenNumber(y); //four
end;