    Return the context of the current subroutine call.
    
    Without EXPR, returns "$line $filename".  With EXPR, returns
    "$line $subroutine $filename"; this extra information can be used to
    provide a stack trace.
    
    The value of EXPR indicates how many call frames to go back before the
    current one; the top frame is frame 0.
    
    Exit Status:
    Returns 0 unless the shell is not executing a shell function or EXPR
    is invalid.
