Using an AF Formula as a Counter

I'm trying to create a counter for how many times a tag equals a certain value using AF formulas and attributes (as I'm not allowed to add more PI tags), so this restricts me to AF formula syntax (not PEs).

How can this be done?  For example, using Sinusoid as a test, how can I increment a counter(B) every time sinusoid(A) equals 50 ?

Something like this is needed ?

A=sinusoid

B=counter

Expression:  if (A==50) then B+1 else donothing

I cannot use if (A==50) then B+1 without the else as it gives a syntax error

How do I 'donothing' in AF formula syntax ??

Thanks for the help !