# Look up one of the prefixes associated with a URI in this context.
	#
	# Since many prefixes may be mapped to the same URI,
	# the return value may be unreliable.
	#
	# Parameters:
	#
	# * `uri`: URI to look up.
	#
	# Returns:
	#
	# The associated prefix, or `null` if none is declared.
	#
	# SEE: `NamespaceSupport.prefix`
	fun prefix(uri: String): nullable String do
		# Note: We do not use the original code from SAX 2.0.1 because it is
		# buggy with redefined prefixes. For example, with
		# `<x xmlns:y="1"><z xmlns:y="2" /></x>`, when in `z`, `uri("1")`
		# returns `"y"` in the original code while it should return `null`.
		# Our code is slower, but it works.
		var all_prefixes = prefixes

		for prefix in all_prefixes do
			if uri == self.uri(prefix) then
				return prefix
			end
		end
		return null
	end