Ket TM-extract-text(indices-sp) input-seq
first, it sorts the indices in the indices-sp
where the indices are in range [1, input-seq.size]
if out of this range, they are ignored, ditto if not an integer
elements that correspond to sequential indices are concatenated with a space char
elements that correspond to non sequential indices are concatenated with an underline char
currently the concatination chars are hard-coded in, later they might be set to globals
this operator is a member of the Template Machine set of operators