Renaming or aliasing NH macro causes stack limit error

[Sebastien Peterson-Boudreau]

Working with the ms macros, I'm getting a weird error whenever I try to rename
or alias the NH macro. Here is a minimal example:

        $ groff -ms >/dev/null <<EOF
        .rn NH FOO
        .FOO
        foo
        EOF
        troff:<standard input>:2: fatal error: input stack limit of 1000 levels
        exceeded (probable infinite loop)

        $ groff -ms >/dev/null <<EOF
        .als FOO NH
        .FOO
        foo
        EOF
        troff:<standard input>:2: fatal error: input stack limit of 1000 levels
        exceeded (probable infinite loop)

Any experts know why this happens? Is it expected, or a bug?
-- 
S.