libharu：HPDFPage#concat

everyday Ruby 
#!/usr/bin/env ruby

require "hpdf"

pdf  = HPDFDoc.new
page = pdf.add_page
p page.get_trans_matrix

rad = 45 / 180.0 * Math::PI
page.concat(Math.cos(rad), Math.sin(rad), -Math.sin(rad), Math.cos(rad), 100, 100)
p page.get_trans_matrix

page.move_to(100, 100)
page.line_to(200, 100)

page.move_to(150, 50)
page.line_to(150, 150)
page.stroke

pdf.save_to_file(File.basename($0, ".rb") + ".pdf")

で、

[1.0, 0.0, 0.0, 1.0, 0.0, 0.0]
[0.70710676908493, 0.70710676908493, -0.70710676908493, 0.70710676908493, 100.0, 100.0]

なぜ set_trans_matrix とかでなくて、この名前？