definiens: or sha ll mean any mixture of the -enantiomer with the chemical name ( 2R - cis)-4 - amino-5 - fluoro - l-[2-(hydro xymethyl)-1,3 - oxathiolan-5 - yl]-2(1H)-pyrimidinone a nd the ( + ) enantiomer with the chemical name ( 2S - cis)- 4 - amino-5 - fluoro-1-[2-(hydroxymethyl)-1,3 - oxathiola n-5 - yl]-2(lH)-pyrimidinone, in which the ratio of such ( - ) enantiomer to such ( + ) enantiomer is less than ninety ( 90 ) to ten ( 10 ), and any salts, esters and [ * * * ] d erivatives thereof