CALL MRF(FUNC,XLO,XHI,X0,ERROR,TOL,MAX,IERR) -- This subroutine
 uses modified regula falsi to find a root of the user supplied 
 function.  The root must satisfy both an error bound on X0 and
 a function tolerance on FUNC(X0).  Extrapolation is performed
 if necessary.  FCTR: MRF
 
 FUNC - Function name, real function of a single real variable.  Must
        be declared EXTERNAL in calling program.
 XLO - Lower bound on root.
 XHI - Upper bound on root.
 X0 - Root.
 ERROR - Allowable error on X0.
 TOL - Allowable error on FUNC(X0).
 MAX - Maximum number of iteratons to perform.
 IERR - Return code, where:
      = 0, Root found satisfying all conditions.
      = -1, Did not satisfy error requirement on X0.
      = -2, Did not satisfy error requirement on FUNC(X0).
      = -4, Did not find root in MAX iterations.
      = -8, Extrapolation, X0 < XLO.
      = -16, Extrapolation, X0 > XHI.
 
 Note: This routine works best if FUNC(XLO)*FUNC(XHI) < 0.  The
 returned codes may be summed.