Library /sys$common/syshlp/basichelp.hlb Run time errors, USEABOINP  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  User aborted input, locate point canceled (ERR=293)

  The middle mouse button was pressed during the execution of a graphics
  input  statement  that  uses  a  mouse  to  enter points (e.g.  LOCATE
  POINT).  The pressing of the middle mouse button aborts  the  graphics
  input statement in progress and the data in the variables used for the
  graphics input statement is unchanged.

  The pressing of the  middle  mouse  button  during  a  graphics  input
  statement  is  analogous  to typing CTRL/Z at regular INPUT statement.
  The program can trap this error in an error handler  and  attempt  the
  input statement again if so desired.