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c optional inputs.
c
c the following is a list of the optional inputs provided for in the
c call sequence.  (see also part ii.)  for each such input variable,
c this table lists its name as used in this documentation, its
c location in the call sequence, its meaning, and the default value.
c the use of any of these inputs requires iopt = 1, and in that
c case all of these inputs are examined.  a value of zero for any
c of these optional inputs will cause the default value to be used.
c thus to use a subset of the optional inputs, simply preload
c locations 5 to 10 in rwork and iwork to 0.0 and 0 respectively, and
c then set those of interest to nonzero values.
c
c name    location      meaning and default value
c
c h0      rwork(5)  the step size to be attempted on the first step.
c                   the default value is determined by the solver.
c
c hmax    rwork(6)  the maximum absolute step size allowed.
c                   the default value is infinite.
c
c hmin    rwork(7)  the minimum absolute step size allowed.
c                   the default value is 0.  (this lower bound is not
c                   enforced on the final step before reaching tcrit
c                   when itask = 4 or 5.)
c
c ixpr    iwork(5)  flag to generate extra printing at method switches.
c                   ixpr = 0 means no extra printing (the default).
c                   ixpr = 1 means print data on each switch.
c                   t, h, and nst will be printed on the same logical
c                   unit as used for error messages.
c
c mxstep  iwork(6)  maximum number of (internally defined) steps
c                   allowed during one call to the solver.
c                   the default value is 500.
c
c mxhnil  iwork(7)  maximum number of messages printed (per problem)
c                   warning that t + h = t on a step (h = step size).
c                   this must be positive to result in a non-default
c                   value.  the default value is 10.
c
c mxordn  iwork(8)  the maximum order to be allowed for the nonstiff
c                   (adams) method.  the default value is 12.
c                   if mxordn exceeds the default value, it will
c                   be reduced to the default value.
c                   mxordn is held constant during the problem.
c
c mxords  iwork(9)  the maximum order to be allowed for the stiff
c                   (bdf) method.  the default value is 5.
c                   if mxords exceeds the default value, it will
c                   be reduced to the default value.
c                   mxords is held constant during the problem.
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