Raise exception from `Range#first` with beginless ranges.
MODIFY
mrbgems/mruby-range-ext/mrblib/range.rb
mrbgems/mruby-range-ext/mrblib/range.rb
@@ -11,6 +11,7 @@ class Range
# (10..20).first(3) #=> [10, 11, 12]
#
def first(*args)
+ raise RangeError, "cannot get the first element of beginless range" if self.begin.nil?
return self.begin if args.empty?
raise ArgumentError, "wrong number of arguments (given #{args.length}, expected 1)" unless args.length == 1

Raise exception from `Range#first` with beginless ranges.