docs: def findCredential():
      """
      Follow the usual path that GSI libraries would
      follow to find a valid proxy credential but
      also allow an end entity certificate to be used
      along with an unencrypted private key if they
      are pointed to by X509_USER_CERT and X509_USER_KEY
      since we expect this will be the output from
      the eventual ligo-login wrapper around
      kinit and then myproxy-login.
      """
  
      # use X509_USER_PROXY from environment if set
      if os.environ.has_key('X509_USER_PROXY'):
          filePath = os.environ['X509_USER_PROXY']
          if validateProxy(filePath):
              return filePath, filePath
          else:
              RFCproxyUsage()
              sys.exit(1)
  
      # use X509_USER_CERT and X509_USER_KEY if set
      if os.environ.has_key('X509_USER_CERT'):
          if os.environ.has_key('X509_USER_KEY'):
              certFile = os.environ['X509_USER_CERT']
              keyFile = os.environ['X509_USER_KEY']
              return certFile, keyFile
  
      # search for proxy file on disk
      uid = os.getuid()
      path = "/tmp/x509up_u%d" % uid
  
      if os.access(path, os.R_OK):
          if validateProxy(path):
              return path, path
          else:
              RFCproxyUsage()
              sys.exit(1)
  
      # if we get here could not find a credential
      RFCproxyUsage()
      sys.exit(1)