8.6.1.1090 D2* d-two-star DOUBLE
( xd1 -- xd2 )
xd2 is the result of shifting xd1 one bit toward the most-significant bit, filling the vacated least-significant bit with zero.
Testing:
T{ 0. D2* -> 0. D2* }T
T{ MIN-INT 0 D2* -> 0 1 }T
T{ HI-2INT D2* -> MAX-2INT 1. D- }T
T{ LO-2INT D2* -> MIN-2INT }T
T{ MIN-INT 0 D2* -> 0 1 }T
T{ HI-2INT D2* -> MAX-2INT 1. D- }T
T{ LO-2INT D2* -> MIN-2INT }T