code: def triple_reference_of(label: ShExJ.tripleExprLabel, cntxt: Context) -> Optional[ShExJ.tripleExpr]:
      
      te: Optional[ShExJ.tripleExpr] = None
      if cntxt.schema.start is not None:
          te = triple_in_shape(cntxt.schema.start, label, cntxt)
      if te is None:
          for shapeExpr in cntxt.schema.shapes:
              te = triple_in_shape(shapeExpr, label, cntxt)
              if te:
                  break
      return te
docs: Search for the label in a Schema