browser()
browser causes a pause in the execution
of the current expression and runs a copy of the R interpreter
which has access to variables local to the environment
where the call took place.
Local variables can be listed with ls,
and manipulated with R expressions typed to this sub-interpreter.
The interpreter copy is exited by typing c.
Execution then resumes at the
statement following the call to
browser.
Typing n causes the step-through-debugger, to start
and it is possible to step through the remainder of
the function one line at a time.
debug, and traceback for the stack an error.