XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (333 page)

The priority of operators is indicated by the grammar. Unary
+
and
-
have a higher priority (bind more tightly) that the multiplicative operators
*
,
div
,
idiv
, and
mod
, which in turn have higher priority than the binary forms of
+
and
-