Fosinopril impurity C – mixture of (2S,4S)-4-cyclohexyl-1-[[(S)-[(1S)-2-methyl-1-(1-oxopropoxy)propoxy](4-phenylbutyl)phosphoryl]acetyl]pyrrolidine-2-carboxylic acid and (2S,4S)-4-cyclohexyl-1-[[(R)-[(1R)-2-methyl-1-(1-oxopropoxy)propoxy](4-phenylbutyl)phosphoryl]acetyl]pyrrolidine-2-carboxylic acid