A Prelude

1. An introductory or preliminary performance or event.

Synonym: preface

2. (music) A short, free-form piece of music, originally one serving as an introduction to a longer and more complex piece; later, starting with the Romantic period, generally a stand-alone piece. [from 1650s]

Synonyms: intrada, overture

3. (computing) A standard module or library of subroutines and functions to be imported, generally by default, into a program.
4. (figurative) A forerunner to anything.

forestory