filtering: Filter signals.
In hadley/shinySignals: Stream functions from shiny.

There are two variations: do you want to use a predicate function (if) or a boolean stream (when); and do you want to keep or drop the values.

keepIf(signal, predicate, init = NULL)

dropIf(signal, predicate, init = NULL)

keepWhen(signal, predicate, init = NULL)

dropWhen(signal, predicate, init = NULL)

`signal`	A signal
`predicate`	For `if` functions, a predicate function; for `when`, a signal yielding TRUE or FALSE.
`init`	Shiny does not allow undefined signals, so an initial value must be provided in case the first signal is filtered out.