Unattested Tibetan phrase

[Stephen Hodge]

As is well known, a number of Mahayana sutras contain descriptions of
pu.skari.nii pools with their stairways leading down to the water.  I have
one such passage which contains the Tibetan phrase " 'drul-ba'i lam" but I
cannot find any mention of 'drul-ba that would make sense in the context
(i.e. it's not a "rotten path") -- it's also not a misprint since several
Kanjura editions have it.  The Chinese parallels to my passage omit it
altogether and the Mongolian doesn't seem to help much either.   The
sentence says that the pools have 'drul-ba'i lam which are furnished with
stairways with golden steps.  From the context I would assume that
'drul-ba'i lam could mean something like a descending ramp.  But has anybody
come across this expression and can suggest a possible Skt form ?

Best wishes,
Stephen Hodge