// set a breakpoint at line 22 in the current program
  program p = this_program;
  add_breakpoint(p, 22);

$ telnet localhost 3333
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Welcome to the Pike Debugger.
Breakpoint on /Users/hww3/bpt.h:2
> backtrace;
(1) Result: ({ /* 4 elements */
                backtrace_frame(/Users/hww3/pike2/build/darwin-16.7.0-x86_64/master.pike:4148, _main(), Args: 1),
                backtrace_frame(-:1, `()(), Args: 1),
                backtrace_frame(/Users/hww3/bpt.pike:14, doit(), No args),
                backtrace_frame(/Users/hww3/bpt.h:2, whee(), Args: 1)
            })
> backtrace[-1]->locals();
(4) Result: ({ /* 3 elements */
                "rad",
                0,
                0
            })
> backtrace[-1]->set_local(1, "bar");
(5) Result: 0
> backtrace[-1]->locals();
(6) Result: ({ /* 3 elements */
                "rad",
                "bar",
                0
            })
> step
Stepping.
Breakpoint on /Users/hww3/bpt.pike:22
> backtrace;
(1) Result: ({ /* 4 elements */
                backtrace_frame(/Users/hww3/pike2/build/darwin-16.7.0-x86_64/master.pike:4148, _main(), Args: 1),
                backtrace_frame(-:1, `()(), Args: 1),
                backtrace_frame(/Users/hww3/bpt.pike:14, doit(), No args),
                backtrace_frame(/Users/hww3/bpt.pike:22, whee(), Args: 1)
            })
> backtrace[-1]->locals();
(2) Result: ({ /* 3 elements */
                "rad",
                "bar",
                "wq"
            })
> go