Business Rule | One-to-One Transformation - E# Parent to its first E# Base

We need your help to Import flat files:
1. Number of Rows: > 100k
2. Organized under E# Parent Members: a few thousands
3. E# is extended

We've been fortunate to be advised to build a BR to generate a one-to-one Transformation Rule file.
- E# Parent to its first E# Base

Is it possible for you SMEs to be generous enough to share with us/Community:
- a sample BR to look up its first E# Base Member of an E# Parent Member
- under extended E#
- how to call that BR in an Import Workflow

Thank you, OS SMEs.