title: First Set Bit
description:

: FIRSTBIT ( number -- firstbit )
        DUP  1 RSHIFT OR
        DUP  2 RSHIFT OR
        DUP  4 RSHIFT OR
        DUP  8 RSHIFT OR
        DUP 16 RSHIFT OR
        DUP  1 RSHIFT XOR
;

explanation:

 Let us consider the 1st set bit. On the first step, DUP 1 RSHIFT OR,
 it gets copied to the bit that follows it. On the 2nd step, it gets
 copies to the two bits that follow the two bits mentioned on the
 previous step. And so on. Finally, all bits below the first set bit
 become set. Then, 1 RSHIFT XOR clears all set bits except the very 1st one.