# Start a new Namespace context.
	#
	# The new context will automatically inherit
	# the declarations of its parent context, but it will also keep
	# track of which declarations were made within this context.
	#
	# Event callback code should start a new context once per element.
	# This means being ready to call this in either of two places.
	# For elements that don't include namespace declarations, the
	# `ContentHandler.start_element` callback is the right place.
	# For elements with such a declaration, it'd done in the first
	# `ContentHandler.start_prefix_mapping` callback.
	# A boolean flag can be used to
	# track whether a context has been started yet. When either of
	# those methods is called, it checks the flag to see if a new context
	# needs to be started. If so, it starts the context and sets the
	# flag. After `ContentHandler.start_element` does that, it always clears
	# the flag.
	#
	# Normally, SAX drivers would push a new context at the beginning
	# of each XML element. Then they perform a first pass over the
	# attributes to process all namespace declarations, making
	# `ContentHandler.start_prefix_mapping` callbacks.
	# Then a second pass is made, to determine the namespace-qualified
	# names for all attributes and for the element name.
	# Finally all the information for the
	# `ContentHandler.start_element` callback is available,
	# so it can then be made.
	#
	# The Namespace support object always starts with a base context
	# already in force: in this context, only the `xml` prefix is
	# declared.
	#
	# SEE: `sax::ContentHandler`
	#
	# SEE: `pop_context`
	fun push_context do
		current_context.decls_ok = false
		context_position += 1

		# Extend the array if necessary.
		if context_position >= contexts.length then
			current_context = new Context
			contexts.push(current_context)
		else
			current_context = contexts[context_position]
		end

		# Set the parent, if any.
		if context_position > 0 then
			current_context.parent = contexts[context_position - 1]
		end
	end