seqtk doesn't support the full range of ambiguity codes.

[hexylena]

E.g. for a test sequence:

>ambig
ACGTMRWSYKVHDBN

We'll see the following bases identified as ambiguous:

#chr    position  base
ambig   5         M
ambig   6         R
ambig   7         W
ambig   8         S
ambig   9         Y
ambig   10        K

It would be great if seqtk could support the following IUPAC ambiguities:

B   C or G or T
D   A or G or T
H   A or C or T
V   A or C or G
N   any base

If I wrote any C at all, I'd make a PR, but as I don't I'll just leave an issue. It looks like the offending line is code like L812 and L438 which checks if the value obtained from bitcnt_table == 2, whereas there are ambiguities with values 3 and 4.

[lh3]

Both cases are intentional. As its name implies, listhet gives the heterozygous positions. B/D/H/V/N are not hets. randbase is intended to work on hets only.