Thermal Desktop (TD) is fully parametric: any input can be specified as a function of almost anything, including outputs. TD also accepts arbitrarily complex co-solved customizations. To accomplish these feats, TD contains both a spreadsheet-like expression system and a co-executed user logic system. 

NEL (network element logic) is in many ways a powerful hybrid of these two systems, allowing complex customizations to be defined as part of basic modeling elements such as Conductors and Pipes.