module Step:sig..end
    Update functions returned by React.S.create and React.E.create
    implicitely create and execute update steps when used without
    specifying their step argument.
    Using explicit React.step values with these functions gives more control on
    the time when the update step is perfomed and allows to perform
    simultaneous primitive signal updates and event
    occurences. See also the documentation about update steps and
    simultaneous events.
typet =React.step
val create : unit -> React.stepcreate () is a new update step.val execute : React.step -> unitexecute step executes the update step.Invalid_argument if step was already executed.