class_docstr: An abstract visitor defined over Expr.
  
  Defines the default dispatch over expressions, and
  implements memoization.
code: def __init__(self):
      self.memo_map = {}
  
imports: from tvm.ir import Op
  from .function import Function, FunctionWithFields
  from .expr import Call, Let, Var, GlobalVar
  from .expr import If, Tuple, TupleGetItem, Constant
  from .expr import RefCreate, RefRead, RefWrite
  from .adt import Constructor, Match, Clause
label_desc: Use image node_id 1 to create a new ExprFunctor object  with example: obj = ExprFunctor()