Very few things in law are truly reducible to some hard, bright-line rule. There are conflicting interests all worthy of consideration and that therefore defy a single objective rule. So even in the presence of extensive precedent, there is usually a grey area requiring case-by-case judgment based on intuitions of fairness and equity.

Say you own an apartment building with a view of the ocean and I own the plot directly between it and the ocean. I threaten to build a wall that blocks your building's view of the ocean unless you buy my plot for 10x more than its market worth. Should that be legal? Say you succumb to my demand. Should you be able to then sue me and recover the excessive payment? I think many would have the intuition that my threat should not be legal and any resulting contract not valid, since it was extracted by duress.

But how about Firefox selling its search bar to Google for billions of dollars, carrying the implicit threat to use a different default provider should Google not pay. That seems more fair than the previous example, but why?

And how do you formulate that difference into a hard rule, without relying on human beings' (i.e. judge and jury) intuitions of fairness and equity?

Here is what the Restatement of Contracts has to say:

  (1) A threat is improper if
     (a) what is threatened is a crime or a tort, or the 
         threat itself would be a crime or a tort if it 
         resulted in obtaining property,
     (b) what is threatened is a criminal prosecution,
     (c) what is threatened is the use of civil process 
         and the threat is made in bad faith, or
     (d) the threat is a breach of the duty of good faith 
         and fair dealing under a contract with the 
         recipient.
  (2) A threat is improper if the resulting exchange is 
      not on fair terms, and
     (a) the threatened act would harm the recipient and 
         would not significantly benefit the party making 
         the threat,
     (b) the effectiveness of the threat in inducing the 
         manifestation of assent is significantly 
         increased by prior unfair dealing by the party 
         making the threat, or
     (c) what is threatened is otherwise a use of power 
         for illegitimate ends.

Look at all of the subjective terminology: "bad faith", "good faith", "fair dealing", "fair terms", "significantly".

And look at (2)(c), "otherwise a use of power for illegitimate ends". What does that even mean? It's basically a surrender, acknowledging that it is impossible to formulate a rule ex ante that totally captures our notions of fairness as they should be applied in every possible situation.

>Very few things in law are truly reducible to some hard, bright-line rule.

My language is a little sloppy here. There are plenty of legal rules that lawyers would call bright-line rules. But they'll usually still fall short of something that can evaluated in software, as a programmer might expect of a rule so-described.

On a flight to London, I once spoke with an intellectual property lawyer about my pet idea for machine-checked law, at least limited to the domain of contract law.

He explained some of the many reasons why that’s an exceedingly difficult problem. Together we reasoned that it would be possible to implement some degree of machine checking and automation, but that an expert human would always need to review the results, because it’s humans who interpret the laws & contracts.

One legal term that has always stumped me is the meaning of "beyond reasonable doubt". It's not a phrase that we use in everyday speech, so its meaning doesn't come readily to mind. Apparently, it means something less than "absolutely certain" and something more than "reasonably certain". What's worse is that judges cannot attempt to define it for the jury. Doing so can lead to a mistrial. https://www.ruleoflaw.org.au/beyond-reasonable-doubt/

A small step towards that is Smart Contracts built on Ethereum.