Render blocks of text one or more times, depending on the value of the key in the current context.
Depending on the value's type, the following actions happen:
Arrayor can.Observe.List - the block is rendered for each item in the array. The context is set to the item within each block rendering.
truthyvalue - the block is rendered with the context set to the value.
falseyvalue - the block is not rendered.
The rendered result of the blocks, block or an empty string is returned.