Error in line 1: while requiring "./spec/crystal_graph_spec.cr"

in spec/crystal_graph_spec.cr:1: while requiring "./spec_helper"

require "./spec_helper"
^

in spec/spec_helper.cr:2: while requiring "../src/crystal_graph"

require "../src/crystal_graph"
^

in src/crystal_graph.cr:1: while requiring "./crystal_graph/*"

require "./crystal_graph/*"
^

in src/crystal_graph/directed_graph.cr:2: undefined constant Graph

 class DirectedGraph < Graph