Simplification of ElasticSearch OR query

Question

I want to write a simple query in ElasticSearch, that in pseudocode looks like this: WHERE X==A and (Y in (Y1, Y2) or Y is undefined). This is the query that I can come up with that works:

GET sales-staging/_search?filter_path=hits.hits._source
{
  "query": {
    "bool": {
      "must": [
        {
          "term": {
            "A": "X"
          }
        }
      ],
      "should": [
        {
          "terms": {
            "B": [
              "Y1", 
              "Y2"
            ]
          }
        },
        {
          "bool": {
            "must_not": [
              {
                "exists": {
                  "field": "C"
                }
              }
            ]
          }
        }
      ],
      "minimum_should_match": 1
    }
  }
}

I expect these documents to match:

{
    "A": "X",
    "B": "Y1"
},
{
    "A": "X"
}

and these ones to not match:

{
    "A": "X",
    "B": "Y3"
},
{
    "A": "Z",
    "B": "Y1"
},
{
    "A": "Z"
}

However, this is a quite hefty multiline query for such a simple query, and I have the feeling I am misusing the minimum_should_match parameter to write a simple OR query. Am I overlooking a simpler way?

I looked at SO:

• Elasticsearch OR query
• Elasticsearch - "OR" query condition using match query and term query and the documentation

And on the documentation pages:

• https://www.elastic.co/guide/en/elasticsearch/guide/current/combining-filters.html
• https://www.elastic.co/guide/en/elasticsearch/guide/current/bool-query.html

But couldn't find the answer there either.