If sga_use is set in the sgwork structure actions field when the hook
returns, Intuition will use the values in the SGWork fields workbuffer,
numchars, bufferpos, and longint; copying the workbuffer to the stringinfo
buffer.  sga_use is set by intuition prior to calling the hook, and must
be cleared by the hook if the changes are to be ignored.  If SGA_USE is
cleared when the hook returns, the string gadget will be unchanged.

If sga_end is set when the hook returns, intuition will deactivate the
string gadget.  in this case, intuition will place the value found in
sgwork structure code field into the intuimessage.code field of the
idcmp_gadgetup message it sends to the application.

If sga_reuse and sga_end are set when the hook returns, intuition will
reuse the input event after it deactivates the gadget.

Starting in V37, the hook may set sga_prevactive or sga_nextactive with
sga_end.  this tells intuition to activate the next or previous gadget
that has the gflg_tabcycle flag set.

If sga_beep is set when the hook returns, intuition will call
displaybeep().  use this if the user has typed in error, or buffer is full.

Set sga_redisplay if the changes to the gadget warrant a gadget redisplay.
Changes to the cursor position require redisplay.