FormatNumber(number, pattern[, symbols]) -> string
Converts the number argument to a string using the format pattern string
from the pattern argument. The symbols argument is a 10-item tuple which
specifies the special characters used from interpreting the format pattern.
The tuple format is (decimal-separator, grouping-separator, infinity,
minus-sign, NaN, percent, per-mille, zero-digit, digit, pattern-separator).
See the XSLT 1.0 Recommendation Section 12.3 for the meanings of these.