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class DBZ(Exception):
    def __str__(self):
        return 'Division By Zero'
class TM(Exception):
    def __str__(self):
        return 'Type Mismatch'
class UI(Exception):
    def __str__(self):
        return 'Unbound Identifier'
def GetHead(content):
    tmp = content[0]
    del content[0]
    return tmp
def Skip():
    symbol = GetHead(content)
    if symbol in '+-*/':
        Skip()
        Skip()
    elif symbol == 'if':
        Skip()
        Skip()
        Skip()
    elif symbol == 'let':
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