Mail merge nesting

A mail merge template is compiled from a flat list of block markers. Every entry in markers reads either "open|loop" or "close|loop", where the text after the pipe is the block name, and block names may repeat. A template compiles when each open is closed by a close carrying the same name, and blocks close in the reverse order they opened, so a block opened inside another must close before the outer one does. Return -1 when the whole list compiles. Otherwise return the position of the first entry the compiler cannot accept, meaning a close that names something other than the innermost block still open, or a close arriving while no block is open. When the list runs out with blocks still open, return the position of the earliest marker still open. An empty list compiles.