SUBROUTINE COMB(N, P, L, C)

c*********************************************************************72
c
cc COMB selects a subset of order P from a set of order N.
c
C THIS SUBROUTINE FINDS THE COMBINATION SET OF N THINGS
C TAKEN P AT A TIME FOR A GIVEN LEXICOGRAPHICAL INDEX.
C N - NUMBER OF THINGS IN THE SET
C P - NUMBER OF THINGS IN EACH COMBINATION
C L - LEXICOGRAPHICAL INDEX OF COMBINATION SOUGHT
C C - OUTPUT ARRAY CONTAINING THE COMBINATION SET
C THE FOLLOWING RELATIONSHIPS MUST EXIST AMONG THE INPUT
C VARIABLES.  L MUST BE GREATER THAN OR EQUAL TO 1 AND LESS
C THAN OR EQUAL TO THE MAXIMUM LEXICOGRAPHICAL INDEX.
C P MUST BE LESS THAN OR EQUAL TO N AND GREATER THAN ZERO.
C
      INTEGER N, P, L, C(P), K, R, P1, BINOM, I

C SPECIAL CASE CODE IF P = 1
      IF(P.EQ.1)THEN
        C(1) = L
        RETURN
      ENDIF

C INITIALIZE LOWER BOUND INDEX AT ZERO
      K = 0
C LOOP TO SELECT ELEMENTS IN ASCENDING ORDER
      P1 = P - 1
      C(1) = 0
      DO 20 I=1,P1
C SET LOWER BOUND ELEMENT NUMBER FOR NEXT ELEMENT VALUE
        IF (I.NE.1) C(I) = C(I-1)
C LOOP TO CHECK VALIDITY OF EACH ELEMENT VALUE
   10   C(I) = C(I) + 1
        R = BINOM(N-C(I),P-I)
        K = K + R
        IF (K.LT.L) GO TO 10
        K = K - R
   20 CONTINUE
      C(P) = C(P1) + L - K

      RETURN
      END
      INTEGER FUNCTION BINOM(M, N)

c*********************************************************************72
c
c
cc BINOM computes the binomial coefficient.
c
C ACM ALGORITHM 160 TRANSLATED TO FORTRAN.  CALCULATES THE
C NUMBER OF COMBINATIONS OF M THINGS TAKEN N AT A TIME.
C
      INTEGER M, N, P, I, N1, R
      N1 = N
      P = M - N1
      IF (N1.GE.P) GO TO 10
      P = N1
      N1 = M - P
   10 R = N1 + 1
      IF (P.EQ.0) R = 1
      IF (P.LT.2) GO TO 30
      DO 20 I=2,P
        R = (R*(N1+I))/I
   20 CONTINUE
   30 BINOM = R

      RETURN
      END