Match fragment piece from white-list

[noahcape]

Motivation: https://teichlab.github.io/scg_lib_structs/methods_html/BD_Rhapsody.html

One can specify f[CAGAGC] to match a sequence in a read. Due to the grammar, if one wants to match a variable length segment not at the end of a read a fragment must right flank a variable length segment. If a white-list of sequences exists to match, one should be able to write f[white-list.txt].