def variable_exists(name)
      assert_type name, :String, :name
      bool(environment.caller.var(name.value))
    end
def variable_exists(name)
      assert_type name, :String, :name
      bool(environment.caller.var(name.value))
    end
Check whether a variable with the given name exists in the current
 scope or in the global scope.

 @example
   $a-false-value: false;
   variable-exists(a-false-value) => true
   variable-exists(a-null-value) => true

   variable-exists(nonexistent) => false

 @overload variable_exists($name)
   @param $name [Sass::Script::Value::String] The name of the variable to
     check. The name should not include the `$`.
 @return [Sass::Script::Value::Bool] Whether the variable is defined in
   the current scope.