Richard Stallman

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. 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:

      ⎕←X←3 1 4∘.+2 7 1 8
5 10 4 11
3  8 2  9
6 11 5 12
      ⍴X
3 4
      ⍴⍴X
2
      ⍴⍴⍴X
1