'Checks all calls. Remembers the caller of the initial format() call, or
in other words, the left-hand side of the call. Also tracks if HTML()
or Text() calls were seen.
Arguments:
node: The AST root node.'

def visit_Call(self, node):

    if (isinstance(node.func, ast.Attribute) and (node.func.attr is 'format')):
        if (self.format_caller_node is None):
            self.format_caller_node = node.func.value
    elif (isinstance(node.func, ast.Name) and (node.func.id in ['HTML', 'Text'])):
        self.interpolates_text_or_html = True
    self.generic_visit(node)
