code: def reference_of(selector: shapeLabel, cntxt: Union[Context, ShExJ.Schema] ) -> Optional[ShExJ.shapeExpr]:
      
      schema = cntxt.schema if isinstance(cntxt, Context) else cntxt
      if selector is START:
          return schema.start
      for expr in schema.shapes:
          if not isinstance(expr, ShExJ.ShapeExternal) and expr.id == selector:
              return expr
      return schema.start if schema.start is not None and schema.start.id == selector else None
docs: Return the shape expression in the schema referenced by selector, if any
  
      :param cntxt: Context node or ShEx Schema
      :param selector: identifier of element to select within the schema
      :return: