private static Op transformFilterSequence(ExprList exprs, Set<Var> varScope, OpSequence opSequence)
{
List<Op> ops = opSequence.getElements() ;
// Any filters that depend on no variables.
Op op = insertAnyFilter(exprs, varScope, null) ;
for ( Iterator<Op> iter = ops.iterator() ; iter.hasNext() ; )
{
Op seqElt = iter.next() ;
// Process the sequence element. This may insert filters (sequence or BGP)
seqElt = transform(exprs, varScope, seqElt) ;
// Merge into sequence.
op = OpSequence.create(op, seqElt) ;
// Place any filters now ready.