Small fixes for SHIFT and KECCAK_ROUND

spec/chapters/shift.typ

@@ -149,9 +149,7 @@ Hereafter, one must only check that `out` is the proper cast of `shifted` into a
 #render_constraint_table(chip, config, groups: "limb_shifting")
 
 == Miscellaneous 
-#render_constraint_table(chip, config, groups: ("left_flag", "is_negative"))
-*Note*: `is_negative` is not used when `signed = 0`.
-As such, there is no problem with it being unconstrained in this case.
+#render_constraint_table(chip, config, groups: ("left_flag", "sign"))
 
 == Lookups
 This chip adds the following interaction to the lookup.

spec/src/keccak_round.toml

@@ -190,7 +190,7 @@ kind = "interaction"
 tag = "KECCAK_RC"
 input = ["round"]
 output = "rc"
-multiplicity = ["-", "μ"]
+multiplicity = "μ"
 
 [[constraint_groups]]
 name = "theta"

spec/src/shift.toml

@@ -45,9 +45,9 @@ pad = 0
 # Auxiliary
 
 [[variables.auxiliary]]
-name = "is_negative"
+name = "sign"
 type = "Bit"
-desc = "Whether `in` is negative"
+desc = "The sign bit of `in`"
 pad = 0
 
 [[variables.auxiliary]]
@@ -95,7 +95,7 @@ def = {idx = "i", polys = [
 name = "extension"
 type = "Half"
 desc = "sign extension of `in`."
-def = ["*", 65535, "is_negative"]
+def = ["*", 65535, "sign"]
 
 [[variables.virtual]]
 name = "left"
@@ -204,15 +204,13 @@ ref = "shift:c:direction_implies_mu"
 
 
 [[constraint_groups]]
-name = "is_negative"
+name = "sign"
 
-[[constraints.is_negative]]
-kind = "interaction"
-tag = "MSB16"
-input = [["idx", "in", 3]]
-output = "is_negative"
-multiplicity = "signed"
-ref = "shift:c:is_negative_if_signed"
+[[constraints.sign]]
+kind = "template"
+tag = "SIGN"
+input = [["idx", "in", 3], "signed"]
+output = "sign"
 
 
 [[constraint_groups]]