( Low-level part of ROLL. Example:
  "1 2 3 4 4 (roll)" --> "1 3 4 4". No sanity checks, never
  call with 0. )
CODE (roll)
    HL POP,
    B H LDrr,
    C L LDrr,
    SP ADDHLd,
    HL INCd,
    D H LDrr,
    E L LDrr,
    HL DECd,
    HL DECd,
    LDDR,
;CODE