[build] add annot flag via tags
[exe] extend simple example to use changeables
[build] _tags not used?
[git] ignore annot files
[make] add obuild DEBUG flag
[examples] tidy up and modularise
[examples] add if/then/else control flow example
[examples] more sensible file naming
[examples] control flow, making the change propagation dynamic
[examples] remove spurious module import
[examples] add memoising previous run example
[examples] clean up invocations