// filterVersions classifies versions into releases and pre-releases, filtering
// out:
//  1. versions that do not satisfy the 'allowed' predicate, and
//  2. "+incompatible" versions, if a compatible one satisfies the predicate
//     and the incompatible version is not preferred.
//
// If the allowed predicate returns an error not equivalent to ErrDisallowed,
// filterVersions returns that error.

if any command fails unexpectedly, no subsequent commands in the script are
executed. The command prefix ! indicates that the command on the rest of the
line (typically go or a matching predicate) must fail instead of succeeding.
The command prefix ? indicates that the command may or may not succeed, but the
script should continue regardless.

The command prefix [cond] indicates that the command on the rest of the line

			// provides syntactic information (which doesn't rely
			// on the order in which the objects are set up). For
			// local objects, we can rely on the order, so use
			// the object's predicate.
			// TODO(gri) It would be less fragile to always access
			// the syntactic information. We should consider storing
			// this information explicitly in the object.
			var alias bool
			if check.conf.EnableAlias {

	pcdata.To.Offset = oldval

	return pcdata
}

// MarkUnsafePoints inserts PCDATAs to mark nonpreemptible and restartable
// instruction sequences, based on isUnsafePoint and isRestartable predicate.
// p0 is the start of the instruction stream.
// isUnsafePoint(p) returns true if p is not safe for async preemption.
// isRestartable(p) returns true if we can restart at the start of p (this Prog)

// if any command fails unexpectedly, no subsequent commands in the script are
// executed. The command prefix ! indicates that the command on the rest of the
// line (typically go or a matching predicate) must fail instead of succeeding.
// The command prefix ? indicates that the command may or may not succeed, but
// the script should continue regardless.
//

// The value returned is the logical OR of multiple possible formats.
// If the value is FormatUnknown, then the input Header cannot be encoded
// and an error is returned explaining why.
//
// As a by-product of checking the fields, this function returns paxHdrs, which
// contain all fields that could not be directly encoded.
// A value receiver ensures that this method does not mutate the source Header.