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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 build_summary(val1=None, val2=None, val3=None):
 """ Constructs the summary given va1, va2, val3 Args: val1: val2: val3: Returns: """

        summary = []
        if val1 is not None:
            summary.append(val1)
        if val2 is not None:
            summary.append(val2)
        if val3 is not None:
            summary.append(val3)
        if not summary:
            return None
        return ' : '.join(summary)