import re


def extract_enums(text):
    """Extract enums from text
    
    Choices should satisfy following conditions:
    - is surrouned by curly braces, 
    - able to evaluated as valid set
    
    Args:
        text (str): text to find choices
        
    Returns:
        Union[None, List]: None if choices are not found, else the List containing possible choices in sorted order
    """
    pat = r'\{[^\{\}]+\}'
    out = None
    for m in re.finditer(pat, text):
        #curly braces expression
        cb = m.group()
        try:
            out = eval(cb)
            assert isinstance(out, set)
            out = sorted(list(out))
        except:
            pass
    return out