code: private function getOperationTypes($schemaDef)
      {
          $opTypes = [];
  
          foreach ($schemaDef->operationTypes as $operationType) {
              $typeName  = $operationType->type->name->value;
              $operation = $operationType->operation;
  
              if (isset($opTypes[$operation])) {
                  throw new Error(sprintf('Must provide only one %s type in schema.', $operation));
              }
  
              if (! isset($this->nodeMap[$typeName])) {
                  throw new Error(sprintf('Specified %s type "%s" not found in document.', $operation, $typeName));
              }
  
              $opTypes[$operation] = $typeName;
          }
  
          return $opTypes;
      }
docs: @param SchemaDefinitionNode $schemaDef
  
  @return string[]
  
  @throws Error