Re^3: [implementation specific to windows] writing a proper batch file for terminal start-up

in reply to Re^2: [implementation specific to windows] writing a proper batch file for terminal start-up
in thread [implementation specific to windows] writing a proper batch file for terminal start-up

I fail to see how _dirs is populated. Can someone talk through how this works? In particular, I do not understand the syntax of the following lines:

:) Basic debugging checklist

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