code: def find_undeclared_variables(ast):
      
      codegen = TrackingCodeGenerator(ast.environment)
      codegen.visit(ast)
      return codegen.undeclared_identifiers
docs: Returns a set of all variables in the AST that will be looked up from
      the context at runtime.  Because at compile time it's not known which
      variables will be used depending on the path the execution takes at
      runtime, all variables are returned.
  
      >>> from jinja2 import Environment, meta
      >>> env = Environment()
      >>> ast = env.parse('{% set foo = 42 %}{{ bar + foo }}')
      >>> meta.find_undeclared_variables(ast) == set(['bar'])
      True
  
      .. admonition:: Implementation
  
         Internally the code generator is used for finding undeclared variables.
         This is good to know because the code generator might raise a
         :exc:`TemplateAssertionError` during compilation and as a matter of
         fact this function can currently raise that exception as well.