fix quirky negative sign expression markup parsing

lib/liquid.rb

@@ -68,7 +68,6 @@ require 'liquid/extensions'
 require 'liquid/errors'
 require 'liquid/interrupts'
 require 'liquid/strainer_template'
-require 'liquid/expression'
 require 'liquid/context'
 require 'liquid/tag'
 require 'liquid/block_body'
@@ -77,6 +76,7 @@ require 'liquid/variable'
 require 'liquid/variable_lookup'
 require 'liquid/range_lookup'
 require 'liquid/resource_limits'
+require 'liquid/expression'
 require 'liquid/template'
 require 'liquid/condition'
 require 'liquid/utils'

lib/liquid/expression.rb

@@ -57,7 +57,8 @@ module Liquid
       'true' => true,
       'false' => false,
       'blank' => '',
-      'empty' => ''
+      'empty' => '',
+      '-' => VariableLookup.parse("-")
     }.freeze
 
     DOT = ".".ord