D2/

( 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

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