When researching the bug for #90, I read in the docs:

f_lineno is the current line number of the frame — writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to f_lineno.

I'm probably the last person to learn about this, but it seems like a useful thing to implement. j is already taken as down, but J is free. It's probably easy to implement, at least assuming the error reporting is good for bad jumps.