pub const UNKNOWN_STATE: char = '#';Expand description
SaProt’s placeholder for an unknown amino acid or unknown structural state.
The alphabet is a full 21x21 product — the 20 standard residues plus this,
crossed with the 20 3Di states plus this — so # is available on either
axis. M# is a known residue with unknown structure (the documented
sequence-only form) and #a is the converse.