##' @param formula A formula that specifies the compartments that
##' define the cases with a disease or that have a specific
##' characteristic (numerator), and the compartments that define
##' the entire population of interest (denominator). The
##' left-hand-side of the formula defines the cases, and the
##' right-hand-side defines the population, for example,
##' \code{I~S+I+R} in a \sQuote{SIR} model (see
##' \sQuote{Examples}). The \code{.} (dot) is expanded to all
##' compartments, for example, \code{I~.} is expanded to
##' \code{I~S+I+R} in a \sQuote{SIR} model (see
##' \sQuote{Examples}). The formula can also contain a condition
##' (indicated by \code{|}) for each node and time step to further
##' control the population to include in the calculation, for
##' example, \code{I ~ . | R == 0} to calculate the prevalence
##' when the recovered is zero in a \sQuote{SIR} model. The
##' condition must evaluate to \code{TRUE} or \code{FALSE} in each
##' node and time step. Please note, if the denominator is zero,
##' the prevalence is \code{NaN}. Additionally, when
##' \code{level=3} (within-node prevalence) and the formula
##' contains a condition that evaluates to \code{FALSE}, the
##' prevalence is also \code{NaN}.
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