Singleton value not recognized as numeric subscript
In particular circumstances, a single item selected by taking a subsequence of a sequence might not be recognized as a numeric value for the purpose of subscripting.
For example, if $i is tail((1,2)) then the expression $x[$i] may use the effective boolean value of $i rather than its numeric value.
The problem arises because the constructor
public SequenceExtent(SequenceExtent<T> ext, int start, int length)
may return a SequenceExtent of length 1 rather than reducing it to a NumericValue.