INT 33 - MS MOUSE v6.0+ - SET ALTERNATE MOUSE USER HANDLER                      
	AX = 0018h
	CX = call mask (see #03174)
	ES:DX -> FAR routine to be invoked on mouse events (see #03175)
Return: AX = status
	    0018h if successful
	    FFFFh on error
Notes:	up to three handlers can be defined by separate calls to this function,
	  each with a different combination of shift states in the call mask;
	  calling this function again with a call mask of 0000h undefines the
	  specified handler (official documentation); specifying the same
	  call mask and an address of 0000h:0000h undefines the handler (real
	  life)
	some versions of the documentation erroneously reverse the order of
	  the bits in the call mask
SeeAlso: AX=000Ch,AX=0014h,AX=0019h


Bitfields for mouse call mask:
Bit(s)	Description	(Table 03174)
 0	call if mouse moves
 1	call if left button pressed
 2	call if left button released
 3	call if right button pressed
 4	call if right button released
 5	call if shift button pressed during event
 6	call if ctrl key pressed during event
 7	call if alt key pressed during event
Note:	at least one of 5-7 must be set


(Table 03175)
Values user handler is called with:
	AX = condition mask (same bit assignments as call mask)
	BX = button state
	CX = cursor column
	DX = cursor row
	SI = horizontal mickey count
	DI = vertical mickey count
Return: registers preserved
Note:	in text modes, the row and column will be reported as a multiple of
	  the cell size, typically 8x8 pixels