Richard Stallman

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Richard Matthew Stallman, founder of the GNU Project, was an early adopter of APL, and used it to write a text editor as a high school student in the summer of 1969[1]. The same year, he also wrote a now famous rhyme:

Rho, rho, rho of X
Always equals 1
Rho is dimension, rho rho rank.
APL is fun!

This plays on the fact that X gives the shape (length of each axis; "dimension") of X while ⍴⍴X gives the rank (number of axes/dimensions) and since the number of axes is always a count, it is a single number, which has the shape 1 if a vector, and always returns a vector:

      X3 1 4∘.+2 7 1 8
5 10 4 11
3  8 2  9
6 11 5 12
3 4


  1. Stallman, Richard M. "RMS Berättar".