issue #11 : allowing non-zero exit command
issue #13 : implement equality, addition, and if-then-else expressions
issue #13 : add test-cases expression{2,3,4}
issue #13 : implement expression 'not'; update expression 'add' to allow references; update external-command-value to allow basic value to be used in the string interpolation
issue #13 : implement matching-regular-expression; add a use-case of regexp
issue