// they must appear near the top of the file, preceded
// only by blank lines and other line comments.
//
+// To distinguish build constraints from package documentation, a series of
+// build constraints must be followed by a blank line.
+//
// A build constraint is evaluated as the OR of space-separated options;
// each option evaluates as the AND of its comma-separated terms;
// and each term is an alphanumeric word or, preceded by !, its negation.