This function determines whether a link operation is valid at the current location in the document specified by doc.

The following arguments are optional:

back — if not supplied or 0, the function will test the validity of returning to the position before the previous link operation. If non-zero, it will test whether there is a link to be followed at the current location.

doc — if not supplied or 0, the current document is used. Specify an alternative document with this argument if required.

The function returns 1 if a valid link operation is possible, otherwise it returns 0.