sets a breakpoint at the beginning of the specified procedure or
at the given line. Note: Line number 1 is the first line of a library (for
procedures from libraries), resp. the line with the {.
A line number of -1 removes all breakpoint from that procedure.
Example:
breakpoint(groebner);
==> breakpoint 1, at line 838 in groebner
breakpoint(groebner, 176);
==> breakpoint 2, at line 176 in groebner
breakpoint(groebner, -1);
==> breakpoints in groebner deleted(0x6)