class_docstr: This plan is used for storing information required for probabilistic
  predicate.
  
  Arguments:
      predicate (AbstractExpression): A predicate expression used for
      filtering frames
code: def __str__(self):
      return "AbstractPlan"
  
imports: from evadb.expression.abstract_expression import AbstractExpression
  from evadb.plan_nodes.abstract_scan_plan import AbstractScan
  from evadb.plan_nodes.types import PlanOprType
label_desc: Use image node_id 2 for calling the PPScanPlan obj's underlying member method code with example usage: obj.__str__() and returns: str