Jump to a new position in a script. If a label is specified, control
goes to the first Lab command found that has the same label. If no
label is specified, control goes to the first EndSkip command found.
If the BACK switch is given, the search for a matching Lab or
EndSkip command starts at the beginning of the script; otherwise the
search starts at the Skip command. If a matching Lab/EndSkip is not
found, an error is returned.