Base '*' (zero or more repetitions) on '+' (one or more repetitions).

Base '*' (zero or more repetitions) on '+' (one or more repetitions).

This commit effectively reverts commit 5cf441dc936c16e2264e49038ebe9a108dc750b9
"Base '+' (one or more repetitions) on '*' (zero or more repetitions)."

It turn out, there is a good reason for using '+' as base operation:
'*' can be expressed in terms of '+' as 'r* ::= r+ | <empty>', while
'+' expands as 'r+ ::= r r*' and 'r' is duplicated.

Duplication becomes crucial in presence of tags: if the duplicated
subexpression has tags, then duplication causes an error.