I've read more about FrTime, and the big difference with Cells is that in FrTime *every* value acts as an input cell and every expression is a formula, i.e. everything is built-in, because FrTime redefines all scheme's primitives (this seems quite too intrusive to me). To avoid performance issues, there is an optimizer that "unlifts" constant values and expressions. Another difference is that FrTime is not tied to an OO system.