Function: assert

--Usage--
> assert(condition)

Check that the condition is @true@. Shows a warning message if it is not.

Note: @assert@ is a special built-in keyword, so that the error message can include the condition.

--Parameters--
! Parameter	Type			Description
| @input@	[[type:boolean]]	Condition to check.

--Examples--
> assert(1 + 1 == 2) == nil  # nothing happens
> assert(1 * 1 == 2) == nil  # An error message is shown