For conversion to standard matrix need require 'matrix'

[RobolabGs2]

irb(main):001:0> require 'fast_matrix'
=> true
irb(main):002:0> m = FastMatrix::Matrix[[1, 2], [3, 4]]
Traceback (most recent call last):
        5: from C:/Ruby26-x64/bin/irb.cmd:31:in `<main>'
        4: from C:/Ruby26-x64/bin/irb.cmd:31:in `load'
        3: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        2: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/fast_matrix-0.1.6/lib/matrix/matrix.rb:27:in `inspect'
        1: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/fast_matrix-0.1.6/lib/matrix/matrix.rb:64:in `convert'
NameError (uninitialized constant Matrix)
irb(main):003:0> m.convert
Traceback (most recent call last):
        6: from C:/Ruby26-x64/bin/irb.cmd:31:in `<main>'
        5: from C:/Ruby26-x64/bin/irb.cmd:31:in `load'
        4: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        3: from (irb):3
        2: from (irb):3:in `rescue in irb_binding'
        1: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/fast_matrix-0.1.6/lib/matrix/matrix.rb:64:in `convert'
NameError (uninitialized constant Matrix)
irb(main):004:0> require 'matrix'
=> true
irb(main):005:0> m.convert
=> Matrix[[1.0, 2.0], [3.0, 4.0]]