st: filling in missing string

["Terra Curtis" <terra.curtis@cambridgefinance.com>]

Dear Statalist,

I have a dataset which looks like:

	Product	company	molecule
	A		X		x1
	A		Y		
	A		Z		x1
	.		.		.
	.		.		.
	.		.		.

I have several examples of this throughout a large dataset.  I want to fill
in the missing "x1" under the molecule variable.  Is there some way to do
this using -by product- ?

Thank you!
Terra

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