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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 to_timestamp(arg, format_str, timezone=None):
 """ Parses a string and returns a timestamp. Parameters format_str : A format string potentially of the type '%Y-%m-%d' timezone : An optional string indicating the timezone, i.e. 'America/New_York' Examples -------- Returns ------- parsed : TimestampValue """

    return ops.StringToTimestamp(arg, format_str, timezone).to_expr()