Functions for checking inputs and data structures
in the dem packages, eg demarray.
End users would not normally call these functions directly.
Reduce boilerplate code, and give same – hopefully high quality – error messages across all packages.
Try to provide data that can help with debugging.
Has idea of 'scalar'. Not really part of R - scalar is just a vector of length 1 - but often useful, and gives better error messages.
Functions starting with chk_is_ or err_is_ are checking for
class or type, loosely defined.
When x passes the test,
chk and err functions both
return TRUE.  When x fails the test,
chk functions return a string describing the failure,
and err functions raise an error with that string
as the error message.
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