# Register a local variable.
	# Display an error on toolcontext if a variable with the same name is masked.
	fun register_variable(node: ANode, variable: Variable): Bool
	do
		var name = variable.name
		var found = search_variable(name)
		if found != null then
			self.error(node, "Error: a variable named `{name}` already exists.")
			return false
		end
		scopes.first.variables[name] = variable
		variable.location = node.location
		return true
	end