LEAP

Ask, and you shall be granted.
LEAP is a relational-algebra RDBMS. The front end is command-line, which means that relational algebra operations are encoded using a number of conventions (e.g., operand goes first, selection/join condition - later).

Judging by the demo session, the system is a bit bulky, but may prove useful.

Hat tip to one of the students in my CSC 365 class.