Interrupt option "a" does not always return to prompt

[jdemeyer@…]

Despite what was claimed at #725, interrupt option "a" does not just return to the prompt. Example: start up Singular, press CTRL-C and choose the "a" option:

                     SINGULAR                                 /
 A Computer Algebra System for Polynomial Computations       /   version 4.0.2
                                                           0<
 by: W. Decker, G.-M. Greuel, G. Pfister, H. Schoenemann     \   Feb 2015
FB Mathematik der Universitaet, D-67653 Kaiserslautern        \
> ^C// ** Interrupt at cmd:`$INVALID$` in line:';return();'
abort after this command(a), abort immediately(r), print backtrace(b), continue(c) or quit Singular(q) ?a

I don't see a prompt, it's not clear how to get back at the prompt (in this case, a semi-colon + ENTER works, but not always). Moreover, if I type 1; without a new prompt, I get

                     SINGULAR                                 /
 A Computer Algebra System for Polynomial Computations       /   version 4.0.2
                                                           0<
 by: W. Decker, G.-M. Greuel, G. Pfister, H. Schoenemann     \   Feb 2015
FB Mathematik der Universitaet, D-67653 Kaiserslautern        \
> ^C// ** Interrupt at cmd:`$INVALID$` in line:';return();'
abort after this command(a), abort immediately(r), print backtrace(b), continue(c) or quit Singular(q) ?a
1;
1
   ? abort...
   ? error occurred in or before STDIN line 1: `1;`

[ren]

Will improve documentation.

Another proposed feature: break point after the current command.