Make commenting of builded Expressions more flexible.

[TadeasKucera]

We need a way to comment created Expressions from behind and an option to do it on the same line with a one-line comment. Also when creating conjunctions or disjunctions we would appreciate if it was possible for the comments to be on the same line as the particular terms of the operation and not to place the comments on separate lines. This issue is being implemented in #194.