runjags-class have specialised options available for print, plot and summary. These methods allow a mutate function to be specified which produces additional variables based on the monitor variables. These functions are examples of valid syntax, and may be useful in their own right.
contrasts.mcmc(x, vars)
prec2sd(x, vars)run.jags and can be used as examples of the expected inputs and permitted return values. They must take an MCMC object as input, and return an MCMC object or named list with the same length.
This can be used to add new variables to the posterior chains that are derived from the directly monitored variables in JAGS. This allows the variables to be summarised or extracted as part of the MCMC objects as if they had been calculated in JAGS, but without the computational or storage overheads associated with calculating them in JAGS directly. The contrasts.mcmc and prec2sd functions are examples of valid objects (but both require an argument, so will have to be passed as e.g. mutate=list('contrasts.mcmc', 'variabletocontrast')). See the mutate argument to add.summary.
add.summary for an applciation of these functions.