HHBLITS_AA_TO_ID
============================

Use the hhblits convention to map amino acids to corresponding numbers. Map some undefined single-letter symbols to definite values, such that B can be expressed as aspartic acid or aspartic acid. It is specified that B has the same number as D for aspartic acid (ASP), while J and O have the same number as X for unknown amino acid. U has the same number as C for cysteine (CYS). Z has the same number as E for glutamic acid (GLU), and '-' represents the missing amino acid. Except B, J, O, U, and Z, other letters are numbered in sequence, and their meanings correspond to those in `restype_1to3` .

+------+--------+
| key  | value  |
+======+========+
| A    | 0      |
+------+--------+
| B    | 2      |
+------+--------+
| C    | 1      |
+------+--------+
| D    | 2      |
+------+--------+
| E    | 3      |
+------+--------+
| F    | 4      |
+------+--------+
| G    | 5      |
+------+--------+
| H    | 6      |
+------+--------+
| I    | 7      |
+------+--------+
| J    | 20     |
+------+--------+
| K    | 8      |
+------+--------+
| L    | 9      |
+------+--------+
| M    | 10     |
+------+--------+
| N    | 11     |
+------+--------+
| O    | 20     |
+------+--------+
| P    | 12     |
+------+--------+
| Q    | 13     |
+------+--------+
| R    | 14     |
+------+--------+
| S    | 15     |
+------+--------+
| T    | 16     |
+------+--------+
| U    | 1      |
+------+--------+
| V    | 17     |
+------+--------+
| W    | 18     |
+------+--------+
| X    | 20     |
+------+--------+
| Y    | 19     |
+------+--------+
| Z    | 3      |
+------+--------+
| \-   | 21     |
+------+--------+