8.6.1.1100 D2/ d-two-slash DOUBLE
( xd1 -- xd2 )
xd2 is the result of shifting xd1 one bit toward the least-significant bit, leaving the most-significant bit unchanged.
Testing:
T{ 0. D2/ -> 0. }T
T{ 1. D2/ -> 0. }T
T{ 0 1 D2/ -> MIN-INT 0 }T
T{ MAX-2INT D2/ -> HI-2INT }T
T{ -1. D2/ -> -1. }T
T{ MIN-2INT D2/ -> LO-2INT }T
T{ 1. D2/ -> 0. }T
T{ 0 1 D2/ -> MIN-INT 0 }T
T{ MAX-2INT D2/ -> HI-2INT }T
T{ -1. D2/ -> -1. }T
T{ MIN-2INT D2/ -> LO-2INT }T