A feature to step behind one generation.

Like 'step', it would be great to have a 'undo-step' button to step behind one generation, if possible. Would help in 'seeing' how the rules work.

Btw thanks for this awesome app, its clean, snappy and fun to play with!

Start by locating the existing “step” control and the state that represents the current generation. Trace how one generation is produced and determine whether prior generations are retained. Done means an “undo-step” control restores the immediately preceding generation and supports the intended visual inspection of the rules.

Written by the indexing model from the issue text.