The specification of the standard function fold-left

 The section "16.2.3 fn:fold-left" of the W3C document "XPath and
XQuery Functions and Operators 3.0", contains this text:

"As a consequence of the function signature and the function calling
rules, a type error occurs if the supplied function $f cannot be
applied to two arguments, where the first argument is either the value
of $zero or the result of a previous application of $f, and the second
is $seq or any trailing subsequence of $seq"

The signature of the function fold-left is specified as:

fn:fold-left($seq as item()*,
                 $zero as item()*,
                 $f as function(item()*, item()) as item()*) as item()*

The above quoted paragraph violates the signature, because the
argument $f, as specified cannot have as its second argument a
sequence -- the type of the second argument of $f must be item(). But
the above quoted paragraph says "...the second is $seq or any trailing
subsequence of $seq".

Exactly the same paragraph is used in the specification of fold-right
-- and in that case the text is correct.

It seems that the paragraph was copied from the fold-right
specification into the specification of fold-left and this led to the
observed error.

Hope this would help and lead to a future correction.

-- 
Cheers,
Dimitre Novatchev
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