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SUBROUTINE CONSU (UNDEF)
C$ (Contour - Undefined Value)
C$ In some contouring applications, it is desirable to be able
C$ to have missing function values on the grid. One cannot
C$ simply set these values to a number which is out of the
C$ contouring interval, since linear interpolation between
C$ such a point and one of its neighbors may generate a
C$ contour line which passes between them. If the contour
C$ scan simply ignores grid triangles with one or more
C$ vertices having a specified value, indicating a missing
C$ value, then the problem is eliminated. The value 'UNDEF'
C$ provided to this routine will be used as a missing value
C$ flag. If it is not supplied explicitly, a default of the
C$ smallest positive floating-point number will be used. This
C$ number is unlikely to match any user-provided function
C$ values, and cannot cause an overflow condition to arise in
C$ an arithmetic comparison. In the case of contouring
C$ COMPLEX arrays, only the REAL part is tested for equality
C$ with 'UNDEF'.
C$ (05-MAR-81)