def __init__(self, p_paradigm, p_tree):
        """
        p_paradigm is the given paradigm (attributes and data)
        p_tree is the query tree
        """
        self.paradigm = p_paradigm
        # Validate that this is a domain
        assert self.getType(p_tree) == 'D'
        # Store the attribute
        self.attribute = p_tree[0]
        self.error = None
        # Check that the requested attribute is available
        try:
            self.paradigm.attributes[self.attribute]
        except KeyError:
            self.error = "I couldn't find this attribute: " + self.attribute
            print self.error