8FUNCTIONS: ifira - CYANA function

Synopsis: ifira(<integer>) = <integer>

The function returns the index of the first atom that belongs to the
residue with the given index, or the special value ifira(nr+1) = na+1.

This function is useful, for instance, to construct a loop over all
atoms of a given residue, i:

  do j ifira(i) ifira(i+1)-1
    print "Atom $anam(j) of residue $rnam(i) $rnum(i)"
  end do
