Is this Boolean Expression in Standard POS form or not

Question

I am trying to find if the expression X.(W'+YZ) is POS form or not.

My thinking:

X = sum term (single literal)
(W'+ YZ) = sum term 
Hence X.(W'+YZ) should be in POS form

Am I correct or not? I am asking because some teachers said YZ is not a literal so the expression is not in POS form, but I don't see anything wrong. It's not like it is (YZ)' (whole bar/compliment).

Please correct me if I am wrong.

Answer 1

I got it. Term W'+YZ is not sum term because this is not sum of literals. YZ is not a literal. Because of this X.(W'+YZ) is not in standard POS form.

But this makes me wonder why are expressions like (ABC), XY, XY', X'Y, X.Y.W' are all in POS form (yes they are). I think that it could be because although XY itself is not a literal, but it is a single term so (XY) could be in both SOP and POS form. Same logic should apply for other expression similar to this.

Anyone correct me if I am wrong.