Why this day exists

Yesterday's program could store facts and calculate with them. It still performed every statement in order.

A useful program must sometimes refuse work, choose one response, or repeat a step.

The danger is not learning a few keywords. The danger is writing a path that looks reasonable but takes the wrong turn at one exact boundary.

A clear branch or loop should survive a pencil trace before it earns a run.

When behavior is wrong, that trace should reveal the first value that went the wrong way.