def primary_prop(sco_type):
    """Returns the "primary" property name for each SCO type"""
    prop = 'value'  # Generic catchall
    if sco_type == 'user-account':
        prop = 'user_id'
    elif sco_type in ['file', 'mutex', 'process',
                      'software', 'windows-registry-value-type']:
        prop = 'name'
    elif sco_type == 'directory':
        prop = 'path'
    elif sco_type == 'autonomous-system':
        prop = 'number'
    elif sco_type == 'windows-registry-key':
        prop = 'key'
    elif sco_type == 'x509-certificate':
        prop = 'serial_number'
    return prop