Does anyone know how can i retrieve the permissions of an user using the Jira Web Service?

Question

• Hi, I'm working on an application that is using the Jira Web Service. When i'm creating an
  issue, i need to assign an user to the issue.
  i want to ask if someone knows how can be checked if an user has permission to be
  assigned to an issue?
  or
  
  if there is any method to retrieve all users that has the Assignable User permission per project?
  Thanks

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