exponential_pdfNE

[IshiguroYoshihiro]

Motivation for this change

This PR add a lemma of property of exponential_pdf that exponential_pdf m x is 0 if x < 0.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[affeldt-aist]

This actually can be proved by

exact: lt0_exponential_pdf.

The lemma already exists.

[affeldt-aist]

This can maybe be closed.