It is not obvious how pronouns should be lemmatized (cf. #276 for Slavic). The UD_English corpus does the following:

Nominative (PRP):

I -> I
you -> you
he -> he
she -> she
it -> it
we -> we
they -> they

Accusative (PRP):

me -> I
you -> you
him -> he
her -> she
it -> it
us -> we
them -> they

Dependent possessive (PRP$):

my -> my (!)
your -> you
his -> he
her -> she
its -> its (!)
our -> we
your -> you
their -> they

The pattern here is that they are normalized to nominative case, except for "my" and "its", which should probably be "I" and "it", respectively.

Independent possessive (PRP, no morphological features): mine, yours, ours, theirs, etc.: no normalization

Reflexive (PRP): myself, yourself, ourselves, yourselves, themselves, etc.: no normalization

WH animate: who, whom, whoever, whomever: no normalization

I am not sure why whom, whomever, the independent possessives, and the reflexives aren't normalized to nominative as well.

There is one token where ’s in Let’s has been lemmatized as us (it should presumably be we for consistency).

That said, the simplest policy may be to use the lemma field only for spelling normalization (#513) and not perform case normalization at all. If the end user wants to map pronouns to nominative case, that is not hard to implement as postprocessing once spelling is consistent.

Thoughts?