except that the options are:
-* 0b0NN index 0 thru 3 to copy from subelement in pos XYZW
-* 0b110 to indicate "skip". this is equivalent to predicate masking
-* 0b100 to indicate "constant 0"
-* 0b101 to indicate "constant 1" (or 1.0)
-* 0b111 is not needed.
+* 0b000 to indicate "skip". this is equivalent to predicate masking
+* 0b001 is not needed (reserved)
+* 0b010 to indicate "constant 0"
+* 0b011 to indicate "constant 1" (or 1.0)
+* 0b1NN index 0 thru 3 to copy from subelement in pos XYZW
Evaluating efforts to encode 12 bit swizzle into less proved unsuccessful: 7^4 comes out to 2,400 which is larger than 11 bits.