def keep_accessible_residues(dssp_rsa, threshold):
    """
     From the output of DSSP we keep only accessible residues which have an RSA
     value > threshold (arbitrary threshold).

     Args:
         dssp_rsa (dict): A dictionary as keys = residue index and value = RSA.
         threshold (int): Relative solvant accessibility threshold.

     Returns:
         dict(dict): Keys are the residue ids and as value their solvant accessible area.
    """
    return {key: val for key, val in dssp_rsa.items() if val > threshold}