subroutine exchange (a, old, new)

      character*(*) a, old, new

c Exchanges occurrences of 'OLD' for 'NEW' within 'A'

c NOTE: This will not work on PC or SUN if LEN(NEW) > LEN(OLD)
c    in order to get around this problem temporarily
c    a restriction has been introduced such that len(new) must
c    be less than or equal len(old)

      character*6 chival

      if (len(new) .gt. len(old))
     2          call abort ('Exchange: len(new)'//chival(len(new))
     2             //' may not exceed len(old)'//chival(len(old)))
      in = index (a, old)
      if (in .eq. 1) then
        if (in+len(old) .eq. len(a)+1) then
          a = new
        else
          a = new//a(in+len(old):len(a))
        end if
        in = index (a, old)
      end if

c now 'in' can no longer have the value 1

      do while (in .ne. 0)
        if (in+len(old) .eq. len(a)+1) then
          a = a(1:in-1)//new
        else
          a = a(1:in-1)//new//a(in+len(old):len(a))
        end if
        in = index (a, old)
      end do
      return
      end