homogeneous representation for commands

[andychu]

The problem is that in ASDL, simple commands, assignment statements, for loops, etc. are all different types. But sometimes we want to treat them uniformly.

They should all have a left and a right position at least? (And maybe more positions after that?)

• came up in Hooks for debugger support? #760 for debugger support
• it affects set -x -- trace assignments
• and coverage/profiling too Profiling support and coverage support #687
• errexit message needs span_id for command

Maybe:

• redirects? Can those be inherited? Another option is to refactor the schema...

[andychu]

related to #882

[andychu]

Maybe a compromise at first: every command has a start_spid and end_spid ? Right now that's not consistent.

There could be more spids after the first two for translation?

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Easier solution: We can just print the line of the start spid. We don't need an end spid.