here's a breakdown of use cases for a required element ( i.e. keyword ) which can occur many times ( i.e. unbounded ). the minimum number of occurrences is 1. we're also allowing an acceptable gco:nilReason as an exception to the requirement rule by simply skipping them during processing.

[shuvosarker22]

          here's a breakdown of use cases for a required element ( i.e. `keyword` ) which can occur many times ( i.e. `unbounded` ). the minimum number of occurrences is 1. we're also allowing an acceptable `gco:nilReason` as an exception to the requirement rule by  simply skipping them during processing. 

<!-- use case 1 (valid) -->
<gmd:keyword gco:nilReason="missing"/>
<gmd:keyword />
<gmd:keyword>
  <gco:CharacterString>Nautical Charting</gco:CharacterString>
</gmd:keyword>  

<!-- use case 2 (valid) -->
<gmd:keyword gco:nilReason="missing"/>

<!-- use case 3 (invalid) -->
<gmd:keyword />

<!-- use case 4 (invalid) -->
<gmd:keyword gco:nilReason="missing"/>
<gmd:keyword />

<!-- use case 5 (valid) -->
<gmd:keyword>
  <gco:CharacterString>Nautical Charting</gco:CharacterString>
</gmd:keyword>  

<!-- use case 6 (valid) -->
<gmd:keyword />
<gmd:keyword>
  <gco:CharacterString>Nautical Charting</gco:CharacterString>
</gmd:keyword> 

<!-- use case 7 (invalid) -->
<gmd:keyword gco:nilReason="invalid nil reason"/>

one easy approach to this problem is to simply complain if the document doesn't have at least 1 valid keyword. anything else is invalid. however, that breaks our rule of considering gco:nilReason

<gmd:keyword>
  <gco:CharacterString>Nautical Charting</gco:CharacterString>
</gmd:keyword> 

another approach would be to check if there's at least 1 valid element or 1 element with an acceptable nilReason?

Originally posted by @rshewitt in GSA/data.gov#4924 (comment)

[rshewitt]

@GSA/data-gov-dev-team delete this. i don't have permission.