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The second example in section 9.4.10 op:gYearMonth-equal is op:gYearMonth-equal(xs:gYearMonth("1978-03"), xs:gYearMonth("1986-03Z")) returns false(). The starting instants are 1986-03-01T00:00:00-05:00 and 1986-03-01T00:00:00Z, respectively. The first xs:gYearMonth should read "1986-03", or the first starting instant should start with 1978.
Thanks for reporting this. Fixed (silently) in the master copy.