[CF-metadata] mass_fraction

[Jonathan Gregory]

> Mass fraction is used in the construction mass_fraction_of_X_in_Y,
> where X is a material constituent of Y. It means the ratio of the
> mass of Y to the mass of X (including Y).
> 
> Please check carefully the occurrence X and Y in the last sentence.
> Shouldn't it read:
> 'It means the ratio of the
> mass of X to the mass of Y (including X).'

Yes, I agree. Jonathan