6.27 alternative grammar question

I do not understand why production 6.27 in the RDF  Resource Description
Framework Model and Syntax specification is

[6.27] alternative    ::= '<rdf:Alt' idAttr? '>' member+ '</rdf:Alt>'
                         | '<rdf:Alt' idAttr? memberAttr? '/>'

Since you must have at least one member shouldn't the memberAttr in the
attribute form have at least one, as:

[6.27] alternative    ::= '<rdf:Alt' idAttr? '>' member+ '</rdf:Alt>'
                         | '<rdf:Alt' idAttr? memberAttr+ '/>'

I thought memberAttr? would mean an optional memberAttr but that for
alternative at least one is required.

thanks

< Frederick