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+.help invf Jun99 "Slalib Package"
+.nf
+
+      SUBROUTINE slINVF (FWDS,BKWDS,J)
+
+     - - - - -
+      I N V F
+     - - - - -
+
+  Invert a linear model of the type produced by the
+  slFTXY routine.
+
+  Given:
+     FWDS    d(6)      model coefficients
+
+  Returned:
+     BKWDS   d(6)      inverse model
+     J        i        status:  0 = OK, -1 = no inverse
+
+  The models relate two sets of [X,Y] coordinates as follows.
+  Naming the elements of FWDS:
+
+     FWDS(1) = A
+     FWDS(2) = B
+     FWDS(3) = C
+     FWDS(4) = D
+     FWDS(5) = E
+     FWDS(6) = F
+
+  where two sets of coordinates [X1,Y1] and [X2,Y1] are related
+  thus:
+
+     X2 = A + B*X1 + C*Y1
+     Y2 = D + E*X1 + F*Y1
+
+  the present routine generates a new set of coefficients:
+
+     BKWDS(1) = P
+     BKWDS(2) = Q
+     BKWDS(3) = R
+     BKWDS(4) = S
+     BKWDS(5) = T
+     BKWDS(6) = U
+
+  such that:
+
+     X1 = P + Q*X2 + R*Y2
+     Y1 = S + T*X2 + U*Y2
+
+  Two successive calls to slINVF will thus deliver a set
+  of coefficients equal to the starting values.
+
+  To comply with the ANSI Fortran standard, FWDS and BKWDS must
+  not be the same array, even though the routine is coded to
+  work on the VAX and most other computers even if this rule
+  is violated.
+
+  See also slFTXY, slPXY, slXYXY, slDCMF
+
+  P.T.Wallace   Starlink   11 April 1990
+
+  Copyright (C) 1995 Rutherford Appleton Laboratory
+  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+
+.fi
+.endhelp