Updating an existing value

Long-running programs such as REPLs and daemons can merge another command line into a value they already hold:

// Print help/version/errors and exit as `parse()` does.
cli.update_from(&argv);

// Return errors to the caller.
cli.try_update_from(&argv)?;

update_from_argv and try_update_from_argv also strip argv0 and apply multicall applet selection.

An update is atomic: if parsing or validation fails, the original value is unchanged. Otherwise:

• Existing values count as present for relationships such as required and conflicts.
• Environment variables and defaults fill only empty fields.
• A collection mentioned in the new argv replaces the old collection; an unmentioned one stays unchanged.
• Selecting a different subcommand replaces the old variant. Selecting the same one merges its fields.

Checks that need the original bytes cannot be rerun for an existing value. Choices, portable validation expressions, and value-conditional relationships therefore apply only to values supplied by the new argv.