SKRDEX    SKRDEX    SKRDEX    SKRDEX    SKRDEX    SKRDEX    SKRDEX    
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SUBROUTINE SKRDEX (IR, ISKWRK, RSKWRK, RD)

Returns the perturbation factor of the IRth reaction,
or changes its value, depending on the sign of IR.

INPUT
IR        - Integer scalar, reaction index;
            IR> 0 gets RD(I) from RSKWRK
            IR< 0 puts RD(I) into RSKWRK
ISKWRK(*) - Integer workspace array; dimension at least LENISK.
RSKWRK(*) - Real    workspace array; dimension at least LENRSK.

If IR< 0:
RD        - Real scalar, perturbation factor for reaction IR.

OUTPUT
If IR> 0:
RD        - Real scalar, perturbation factor for reaction IR.