Question about application of Bayes

I have suc­cess­fully con­fused my­self about prob­a­bil­ity again.

I am de­bug­ging an in­ter­mit­tent crash; it doesn’t hap­pen ev­ery time I run the pro­gram. After much con­fu­sion I be­lieve I have traced the prob­lem to a spe­cific line (ac­ti­vat­ing my de­bug log­ger, as it hap­pens; irony...) I have tested my pro­gram with and with­out this line com­mented out. I find that, when the line is ac­tive, I get two crashes on seven runs. Without the line, I get no crashes on ten runs. In­tu­itively this seems like ev­i­dence in favour of the hy­poth­e­sis that the line is caus­ing the crash. But I’m con­fused on how to set up the equa­tions. Do I need a prob­a­bil­ity dis­tri­bu­tion over crash fre­quen­cies? That was the solu­tion the last time I was con­fused over Bayes, but I don’t un­der­stand what it means to say “The prob­a­bil­ity of hav­ing the line, given crash fre­quency f”, which it seems I need to know to calcu­late a new prob­a­bil­ity dis­tri­bu­tion.

I’m go­ing to go with my in­tu­ition and code on the as­sump­tion that the de­bug log­ger should be ac­ti­vated much later in the pro­gram to avoid a race con­di­tion, but I’d like to un­der­stand this math.