<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_all_cwlkeys(items, default_keys=None):
 """Retrieve cwlkeys from inputs, handling defaults which can be null. When inputs are null in some and present in others, this creates unequal keys in each sample, confusing decision making about which are primary and extras. """

    if default_keys:
        default_keys = set(default_keys)
    else:
        default_keys = set(["metadata__batch", "config__algorithm__validate",
                            "config__algorithm__validate_regions",
                            "config__algorithm__validate_regions_merged",
                            "config__algorithm__variant_regions",
                            "validate__summary",
                            "validate__tp", "validate__fp", "validate__fn",
                            "config__algorithm__coverage", "config__algorithm__coverage_merged",
                            "genome_resources__variation__cosmic", "genome_resources__variation__dbsnp",
                            "genome_resources__variation__clinvar"
        ])
    all_keys = set([])
    for data in items:
        all_keys.update(set(data["cwl_keys"]))
    all_keys.update(default_keys)
    return all_keys