adds double-stem arrows (looks like equal signs)

src/features/calt.fea

@@ -62,11 +62,17 @@
   leftArrow
   leftLongArrow
   leftLongArrow2
+  leftDoubleArrow
+  leftLongDoubleArrow
   rightArrow
   rightLongArrow
   rightLongArrow2
+  rightDoubleArrow
+  rightLongDoubleArrow
   leftRightArrow
   leftRightLongArrow
+  leftRightDoubleArrow
+  leftRightLongDoubleArrow
   colon
   approxequal
   asciitilde
@@ -107,11 +113,17 @@
   leftArrow.case
   leftLongArrow.case
   leftLongArrow2.case
+  leftDoubleArrow.case
+  leftLongDoubleArrow.case
   rightArrow.case
   rightLongArrow.case
   rightLongArrow2.case
+  rightDoubleArrow.case
+  rightLongDoubleArrow.case
   leftRightArrow.case
   leftRightLongArrow.case
+  leftRightDoubleArrow.case
+  leftRightLongDoubleArrow.case
   colon.case
   approxequal.case
   asciitilde.case

src/features/liga.fea

@@ -8,23 +8,60 @@
   endash.case
   emdash.case
 ];
+@DASH_ANY = [ @DASH @DASH_CASE ];
 
+# --->
+sub @DASH @DASH @DASH greater by rightLongArrow2;
+sub @DASH_ANY @DASH_ANY @DASH_ANY greater.case by rightLongArrow2.case;
+# -->
+sub @DASH @DASH greater by rightLongArrow;
+sub @DASH_ANY @DASH_ANY greater.case by rightLongArrow.case;
+# [endash emdash]-> (legacy)
+sub [endash emdash] greater by rightLongArrow;
+sub [endash emdash endash.case emdash.case] greater.case by rightLongArrow.case;
 # ->
 sub hyphen greater by rightArrow;
-sub [endash emdash] greater by rightLongArrow;  # legacy
+sub [hyphen hyphen.case] greater.case by rightArrow.case;
-sub @DASH @DASH greater by rightLongArrow;
+# <---
-sub @DASH @DASH @DASH greater by rightLongArrow2;
-# <-
-sub less @DASH @DASH by leftLongArrow;
 sub less @DASH @DASH @DASH by leftLongArrow2;
-ignore sub less' hyphen' @Numeral; # ignore e.g. "<-3" ("less than negative")
+sub less.case @DASH_ANY @DASH_ANY @DASH_ANY by leftLongArrow2.case;
+# <--
+sub less @DASH @DASH by leftLongArrow;
+sub less.case @DASH_ANY @DASH_ANY by leftLongArrow.case;
+# <-[endash emdash] (legacy)
+sub less [endash emdash] by leftLongArrow;
+sub less.case [endash emdash endash.case emdash.case] by leftLongArrow.case;
+# ignore e.g. "<-3" ("less than negative")
+ignore sub less' hyphen' @Numeral;
+ignore sub less.case' [hyphen hyphen.case]' @Numeral;
+# <-
 sub less' hyphen' by leftArrow;
-sub less [endash emdash] by leftLongArrow;  # legacy
+sub less.case' [hyphen hyphen.case]' by leftArrow.case;
 # <->
 sub less rightArrow by leftRightArrow; #  <,->
+sub [less less.case] rightArrow.case by leftRightArrow.case;
 sub leftArrow greater by leftRightArrow; # <-,-
+sub leftArrow.case [greater greater.case] by leftRightArrow.case;
+# <-->
 sub less rightLongArrow by leftRightLongArrow; # <,-->
+sub [less less.case] rightLongArrow.case by leftRightLongArrow.case;
 sub leftLongArrow greater by leftRightLongArrow; # <--,>
+sub leftLongArrow.case [greater greater.case] by leftRightLongArrow.case;
+# <=>
+sub less equal greater by leftRightDoubleArrow;
+sub less.case [equal equal.case] [greater greater.case] by leftRightDoubleArrow.case;
+sub less equal equal greater by leftRightLongDoubleArrow;
+sub less.case [equal equal.case] [equal equal.case] [greater greater.case] by leftRightLongDoubleArrow.case;
+# =>
+sub equal equal greater by rightLongDoubleArrow;
+sub [equal equal.case] [equal equal.case] greater.case by rightLongDoubleArrow.case;
+sub equal greater by rightDoubleArrow;
+sub [equal equal.case] greater.case by rightDoubleArrow.case;
+# <==
+sub less equal equal by leftLongDoubleArrow;
+sub less.case [equal equal.case] [equal equal.case] by leftLongDoubleArrow.case;
+# Note: We do NOT (sub less equal by leftDoubleArrow) as that would
+# interfere with "<=" (less than or equal to)
 
 # x surrounded by numbers, e.g. 3x4 -> 3×4
 sub @Numeral x' @Numeral                                                 by multiply; # 3x9