I'm trying to see why a single (indexable) stack for both continuations AND environment is so different from having the two separated. Also compare the forth 2 stacks with the E and K stacks from the CEK machine. http://www.cs.utah.edu/~mflatt/past-courses/cs6520/public_html/s00/secd.ps http://www.cs.utah.edu/plt/publications/pllc.pdf 6.4 p73 http://lambda-the-ultimate.org/node/2423 Maybe I should implement a CEK machine and see how it behaves compared to Forth? I.e. identify where data sharing is introduced. Maybe the key is in 'let' : it introduces an element in the environment E, but leaves the continuation intact. Which operation leaves the environment intact but changes the continuation?