`val if_statement : Javascript.expression -> Javascript.location -> (Javascript.statement * Javascript.location) -> bool -> (Javascript.statement * Javascript.location) -> bool -> (Javascript.statement * Javascript.location) list`
`val block : (Javascript.statement * Javascript.location) list -> Javascript.statement * Javascript.location`
`val unblock : (Javascript.statement * Javascript.location) -> (Javascript.statement * Javascript.location) list`