fix preparser for certain illegal input
trac #16731 removed unused variables ; only one import per line ; more doc
Merge branch 'public/ticket/16731' of ssh://trac.sagemath.org:22/sage into 16731
trac #16731 brooming things
Trac #16731: Uncaught preparser syntaxerror

This (+return) crashes Sage with a preparser error:
{{{
sage: R.<x> = #note the final space
}}}

URL: http://trac.sagemath.org/16731
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): Frédéric Chapoton