def valid_field_var(var):
return var in phyvars.FIELD or var in phyvars.FIELD_EXTRA<EOL>
Whether a field variable is defined.

    This function checks if a definition of the variable exists in
    :data:`phyvars.FIELD` or :data:`phyvars.FIELD_EXTRA`.

    Args:
        var (str): the variable name to be checked.
    Returns:
        bool: True is the var is defined, False otherwise.