SKRAEX    SKRAEX    SKRAEX    SKRAEX    SKRAEX    SKRAEX    SKRAEX    
          **********************************************
                    **************************
                              ******

SUBROUTINE SKRAEX (IR, ISKWRK, RSKWRK, RA)

Returns the Pre-exponential rate constant
(or sticking coefficient) of the IRth reaction, or changes its
value, depending on the sign of IR.

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

If IR< 0:
RA        - Real scalar, pre-exponential or sticking coefficient for
            reaction IR.
               cgs units, mole-cm-sec-K for pre-exponential,
                          none for sticking coefficients

OUTPUT
If IR> 0:
RA        - Real scalar, pre-exponential or sticking coefficient for
            reaction IR.
               cgs units, mole-cm-sec-K for pre-exponential,
                          none for sticking coefficients