subroutine fpinst(iopt,t,n,c,k,x,l,tt,nn,cc,nest)
c given the b-spline representation (knots t(j),j=1,2,...,n, b-spline
c coefficients c(j),j=1,2,...,n-k-1) of a spline of degree k, fpinst
c calculates the b-spline representation (knots tt(j),j=1,2,...,nn,
c b-spline coefficients cc(j),j=1,2,...,nn-k-1) of the same spline if
c an additional knot is inserted at the point x situated in the inter-
c val t(l)<=x2*k or l