Add 'Assert' consistency to algorithm format check

[benjie]

We have a mixture of - Assert: blah and - Assert blah. We should standardize on one pattern; this adds it to the spec format checks. (It also fixes some minor bugs in the script itself.)

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[martinbonnin]

Nit: I like it better without the : since we don't have Return: foo but Return foo and more generally I find that the spec are usually closer to natural language but consistency > everything so I'm fine with everything and support enforcing this in the format check 👍

[benjie]

Happy to change to the reverse if the group agrees. I favour Assert: because assertions are kind of optional/documentation steps - they should hold already and should never trigger unless there's a bug - so they could theoretically be omitted from the algorithm. The same cannot be said for a "return" statements 😉

They're more like Note: ... to me than Return ....

[leebyron]

love this, thanks for adding the check