Change accidental Ml:Z tag to Ml:B:C,.

Also explicitly forbid probabilities summing to more than 1.0.

SAMtags.tex

@@ -607,11 +607,11 @@ \subsection{Base modifications}
 
 If the above is rewritten in the multiple-modification form, the probabilities are interleaved in the order presented, giving ``{\tt Mm:Z:C+mh,5,12;  Ml:B:C,204,26,89,130}''.
 Note where several possible modifications are presented at the same site, the {\tt Ml} values represent the absolute probabilities of the modification call being correct and not the relative likelihood between the alternatives.
-These need not sum to 256 ($p \approx 1.0$) as the remainder represents the probability that none of the modification types are present.
+These probabilities should not sum to above 1.0 ($\approx 256$ in integer encoding, allowing for some minor rounding errors), but may sum to a lower total with the remainder representing the probability that none of the listed modification types are present.
 In the example used above, the 6th {\tt C} has 80\% chance of being {\tt 5mC}, 10\% chance of being {\tt 5hmC} and 10\% chance of being an unmodified {\tt C}.
 
 {\tt Ml} values for ambiguity codes give the probability that the modification is one of the possible codes compatible with that ambiguity code.
-For example {\tt Mm:Z:C+C,10; Ml:Z:229} indicates a C call with a probability of 90\% of having some form of unspecified modification.
+For example {\tt Mm:Z:C+C,10; Ml:B:C,229} indicates a C call with a probability of 90\% of having some form of unspecified modification.
 
 \end{description}