title: Alternate fields in HTTP POST request body
body: <p>I've been looking for the best way to specify a body type for an HTTP POST request that has two fields that are mutually exclusive.
  I'm using swagger for the API specification, and I have one request that must have either field_A or field_B, but not both.</p>
  
  <p>Right now I am setting both fields as optional, describing the behaviour in the spec (when to set one field or the other), and validating the input on the server. If none of the fields is filled I am returning a 400-Bad-request.</p>
  
  <p>My question is whether or not this is the correct approach (would it be more RESTful if I specify an extra content-type?), or if there is something in Swagger that allows me to make this behaviour more obvious.</p>