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`columns` for filters

Given a record or table, produce a list of its columns' names.

> columns {flags}

input	output
record	list<string>
table	list<string>

Get the columns from the record

> { acronym:PWD, meaning:'Print Working Directory' } |columns
╭───┬─────────╮
│0│acronym│
│1│meaning│
╰───┴─────────╯

Get the columns from the table

> [[name,age,grade]; [bill,20,a]] |columns
╭───┬───────╮
│0│name│
│1│age│
│2│grade│
╰───┴───────╯

Get the first column from the table

> [[name,age,grade]; [bill,20,a]] |columns|first

Get the second column from the table

> [[name,age,grade]; [bill,20,a]] |columns|select1

This is a counterpart to values, which produces a list of columns' values.