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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 combine_first(self, other):
 """ Combine Series values, choosing the calling Series's values first. Parameters other : Series The value(s) to be combined with the `Series`. Returns ------- Series The result of combining the Series with the other object. See Also -------- Series.combine : Perform elementwise operation on two Series using a given function. Notes ----- Result index will be the union of the two indexes. Examples -------- 0 1.0 1 4.0 dtype: float64 """

        new_index = self.index.union(other.index)
        this = self.reindex(new_index, copy=False)
        other = other.reindex(new_index, copy=False)
        if is_datetimelike(this) and not is_datetimelike(other):
            other = to_datetime(other)

        return this.where(notna(this), other)