def which(&block)
      WhichQuery.new(self, &block).exec
    end
a simple query DSL for accessing where(), inspired by gem "squeel"
e.g.:
df.which{ `FamilySize` == `FamilySize`.max }
equals
df.where( df['FamilySize'].eq( df['FamilySize'].max ) )

e.g.:
df.which{ (`NameTitle` == 'Dr') & (`Sex` == 'female') }
equals
df.where( df['NameTitle'].eq('Dr') & df['Sex'].eq('female') )