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diff --git a/sys/plio/plempty.x b/sys/plio/plempty.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<plio.h>
+
+# PL_EMPTY -- Test whether a mask is empty, i.e., contains no nonzero pixels.
+
+bool procedure pl_empty (pl)
+
+pointer	pl			#I mask descriptor
+int	i
+
+begin
+	# Mask is empty if all lines are empty.
+	do i = 1, PL_NLP(pl)
+	    if (PL_LP(pl,i) != PL_EMPTYLINE)
+		return (false)
+
+	return (true)
+
+	# The following also works, but the call to pl_compress is an
+	# unintended side effect which it is best to avoid.
+
+	# call pl_compress (pl)
+	# return (PL_LLOP(pl) == LP_BLEN(Ref(pl,0)))
+end