Editorial problem in Section 4.2 of digital signature spec

In the first paragraph of Section 4.2 of the 4/19 draft, the following sentence appears:

While we specify a mandatory and optional to implement SignatureMethod algorithms, user specified algorithms are permitted.

It is not at all clear what this sentence means. It appears to be grammatically incorrect and may be a typo. I cannot suggest how to fix it, because I do not understand what it's trying to say, but I'm pretty sure it is somehow wrong. 
-- 

+-----------------------+------------------------+-------------------+
| Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
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Received on Sunday, 15 July 2001 12:13:16 UTC