For example, he is very particular about what he eats and drinks, even his water cannot be just any water.
Zeke Zalcstein worked, before he retired a few years ago, on the boundary between mathematics and computational complexity.
He started his career in mathematics, and then Zeke moved into computational complexity theory.
As usual the There is a simple linear time algorithm for the word problem.
The algorithm uses a pushdown store, which is initially empty.