class Sequel::Schema::Generator

def initialize(db, &block)

in the context of this object.
Set the database in which to create the table, and evaluate the block 
def initialize(db, &block)
  @db = db
  @columns = []
  @indexes = []
  @constraints = []
  @primary_key = nil
  instance_eval(&block) if block
  @columns.unshift(@primary_key) if @primary_key && !has_column?(primary_key_name)
end