InvokeProcessor is used to invoke a Processor.

Process performs the entire pipeline process for selection.

RegistProcessor is used to register a Processor with the registry.

ErrAlreadyExists means processor already exists.

ErrUndefinedMethod means method undefined.

ErrWrongArgNumber means wrong args number.

InvokePlaceholder is used for its own replacement.

Pipeline structure is the calling unit in pipeline.

Processor is the function call unit of pipeline.

Callee defines the function body of Processor.

Pipelines defines the entire pipeline process of a selection.