class_docstr: Checker for ast <-> pattern.
  
  NodeVisitor is needed for specific behavior checker.
  
  Attributes
  ----------
  node : AST
      node we want to compare with pattern
  placeholders : [AST]
      list of placeholder value for later comparison or replacement.
class_docstr_tok: ["Checker","for","ast","<","-",">","pattern",".","NodeVisitor","is","needed","for","specific","behavior","checker",".","Attributes","--","--","--","--","--","node",":","AST","node","we","want","to","compare","with","pattern","placeholders",":","[","AST","]","list","of","placeholder","value","for","later","comparison","or","replacement","."]
docstr: Initialize attributes. 
docstr_tok: ["Initialize","attributes","."]
code: def __init__(self, node, placeholders):
      """Initialize attributes."""
      self.node = node
      self.placeholders = placeholders
  
imports: from gast import AST, iter_fields, NodeVisitor, Dict, Set
  from itertools import permutations
  from math import isnan
label_desc: Use image node_id 1 to create a new Check object from inherited base classes: NodeVisitor with example: obj = Check(node, placeholders)