# Return one of the prefixes mapped to a Namespace URI.
	#
	# If more than one prefix is currently mapped to the same
	# URI, this method will make an arbitrary selection; if you
	# want all of the prefixes, use the `prefixes_of` method instead.
	#
	# Note: this will never return the empty (default) prefix;
	# to check for a default prefix, use the `uri`
	# method with an argument of `""`.
	#
	# Parameters:
	#
	# * `uri`: Namespace URI.
	#
	# Returns:
	#
	# One of the prefixes currently mapped to the URI supplied,
	# or `null` if none is mapped or if the URI is assigned to
	# the default Namespace.
	#
	# SEE: `prefixes_of`
	#
	# SEE: `uri`
	fun prefix(uri: String): nullable String do
		return current_context.prefix(uri)
	end