def variable_list(expreStr):
    """get the set of variable of expression.
    Args:
        expreStr: the mba expression string.
    Return:
        variableList: the list of variables.
    """
    varSet = set(expreStr)
    variableList = []
    for i in varSet:
        #the variable name
        if i in ["x", "y", "z", "t", "a", "b", "c", "d", "e", "f"]:
            variableList.append(i)

    return variableList