Optional Declaration of (External) Input Signals

[Raffzahn]

Ist:

Right now any signal used as input for a term is implied 'created'. There should be a way to add an optional definition for every (external) input signal.
An external signal is one that is not defined as output of any other term.

Soll:

Define a syntax to declare external input signals. This opens a way to check for undefined signals instead of automatic creation. Every signal name used for input of a term has to be either the output of a another (former) term or has to be declared as input.

{ assign:
 [
  ["a"],
  ["b"],
  ["out",
    ["OR",
      ["AND",  ["INV", "a"], "b"],
      ["NAND", ["INV", "b"], "a"]
    ]
  ]
]
}