OpenGL man pages
glIndexMask - control the writing of individual bits in the color index
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 ones.
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. Wherever a one appears in
the mask, the corresponding bit in the color index buffer (or buffers) is
made writable. Where a zero appears, the 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, glStencilMask
Fri Dec 6 11:18:03 EST 1996
Look here for legal stuff: Legal