Sets an action to be done before the
executed. The following steps are taken:
Checks for a breakpoint at this line.
Prints the line if necessary.
Performs any actions associated with the line.
Prompts the user if at a breakpoint or in single-step mode.
Evaluates the line.
For example, the following prints the value of
") every time line 53 is passed:
a 53 print "DB FOUND $foo\n"