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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def eval_expression(value):
 """ Evaluate a full time expression """

    start = eval_function(value.ts_expression[0])
    interval = eval_interval(value.ts_expression[2])
    op = value.ts_expression[1]
    if op == "+":
        return start + interval
    elif op == "-":
        return start - interval
    else:
        raise SyntaxError("Unrecognized operator %r" % op)