title: What is '$add' functionality in angularJs?
body: <p>I am new to angularJs, and am in the process of migrating an 0.9 versioned angularJs app to 1.0.5 version. I am stuck at a function named '$add' called on an array. 
  I have searched in the docs for hours, but no use. Can anyone enlighten me regarding this? Also will be nicer to understand the significance/specialty of '$xxx' keyword usage within angular.</p>