fn nt_pos_to_ref_idx(c_pos: isize) -> Option<usize>
0-based index into REF_PNCA of a signed HGVS coding-sequence position. There is no c.0 — c.-1 sits immediately before c.1 (the A of the start codon).
REF_PNCA
c.0
c.-1
c.1
A