glIndexMask -	control	the writing of individual bits in the
color	index buffers

glIndexMask
	glIndexMask(mask) -> None

void glIndexMask( GLuint mask	)

mask	Specifies a bit	mask to	enable and disable the writing
      of individual bits in the color	index buffers.
      Initially, the mask is all 1's.

glIndexMask controls the writing of individual bits in the
color	index buffers.	The least significant n	bits of	mask,
where	n is the number	of bits	in a color index buffer,
specify a mask.  Where a 1 (one) appears in the mask,	it's
possible to write to the corresponding bit in	the color
index	buffer (or buffers). Where a 0 (zero) appears, the
corresponding	bit is write-protected.

This mask is used only in color index	mode, and it affects
only the buffers currently selected for writing (see
glDrawBuffer).  Initially, all bits are enabled for writing.

GL_INVALID_OPERATION is generated if glIndexMask is executed
between the execution	of glBegin and the corresponding
execution of glEnd.

glGet	with argument GL_INDEX_WRITEMASK

glColorMask, glDepthMask, glDrawBuffer, glIndex,
glIndexPointer, glStencilMask