SUBROUTINE INTRV(XT,LXT,X,ILO,ILEFT,MFLAG)
C***BEGIN PROLOGUE  INTRV
C***DATE WRITTEN   800901   (YYMMDD)
C***REVISION DATE  820801   (YYMMDD)
C***CATEGORY NO.  E3,K6
C***KEYWORDS  B-SPLINE,DATA FITTING,INTERPOLATION,SPLINE
C***AUTHOR  AMOS, D. E., (SNLA)
C***PURPOSE  Computes the largest integer ILEFT in 1.LE.ILEFT.LE.LXT
C            such that XT(ILEFT).LE.X where XT(*) is a subdivision
C            of the X interval.
C***DESCRIPTION
C
C     Written by Carl de Boor and modified by D. E. Amos
C
C     Reference
C         SIAM J. Numerical Analysis, 14, No. 3, June 1977, pp. 441-472.
C
C     Abstract
C         INTRV is the INTERV routine of the reference.
C
C         INTRV computes the largest integer ILEFT in 1 .LE. ILEFT .LE.
C         LXT such that XT(ILEFT) .LE. X where XT(*) is a subdivision of
C         the X interval.  Precisely,
C
C                      X .LT. XT(1)                1         -1
C         if  XT(I) .LE. X .LT. XT(I+1)  then  ILEFT=I  , MFLAG=0
C           XT(LXT) .LE. X                         LXT        1,
C
C         That is, when multiplicities are present in the break point
C         to the left of X, the largest index is taken for ILEFT.
C
C     Description of Arguments
C         Input
C          XT      - XT is a knot or break point vector of length LXT
C          LXT     - length of the XT vector
C          X       - argument
C          ILO     - an initialization parameter which must be set
C                    to 1 the first time the spline array XT is
C                    processed by INTRV.
C
C         Output
C          ILO     - ILO contains information for efficient process-
C                    ing after the initial call, and ILO must not be
C                    changed by the user.  Distinct splines require
C                    distinct ILO parameters.
C          ILEFT   - largest integer satisfying XT(ILEFT) .LE. X
C          MFLAG   - signals when X lies out of bounds
C
C     Error Conditions
C         None
C***REFERENCES  C. DE BOOR, *PACKAGE FOR CALCULATING WITH B-SPLINES*,
C                 SIAM JOURNAL ON NUMERICAL ANALYSIS, VOLUME 14, NO. 3,
C                 JUNE 1977, PP. 441-472.
C***ROUTINES CALLED  (NONE)
C***END PROLOGUE  INTRV