(CNN) President Donald Trump reportedly told former FBI Director James Comey, referring to the investigation into the activities of former national security adviser Michael Flynn, that he hoped Comey would "let it go," according to a contemporaneous memo authored by Comey.

Since then, many have suggested that a president asking an FBI director to halt an investigation of a senior campaign adviser constitutes obstruction of justice, and is an impeachable offense.

Obstruction of justice is a broadly defined federal crime. There are several different categories, including obstruction of congressional, administrative , and judicial proceedings, as well as tampering with or retaliating against witnesses.

And that's just obstruction of justice.

There are plenty of other potential federal crimes that might be creatively applied to the president's alleged conduct. That's the beauty of the US Code for US attorneys: there are so many overlapping federal crimes, with such nebulous definitions, that a creative prosecutor can look at almost any conduct and shoehorn it into a few criminal statutes.

There's misprision of a felony, for example.

Mis...what?

That's right; it's a crime most people have never heard of, and one that is rarely charged. Yet, depending on the facts that emerge in the coming months about Trump, Flynn and Comey, it's one that could potentially come into play.

Misprision of felony was a crime under English common law, where "it [was] the duty of a man, who kn[ew] that a felony ha[d] been committed, to report it to the proper authority." In 1790, Congress created a federal misprision statute that is substantially similar to its descendant today (18 U.S.C. § 4)—except the modern statute criminalizes concealmen t, not silence. Mere silence, without some affirmative act, is insufficient evidence of the crime of misprision of felony.

Misprision of a felony occurs when: (1) a third party commits an alleged felony; and (2) the defendant has full knowledge of the crime; (3) he or she fails to notify authorities; and, most importantly, (4) helps conceal the crime. If ordering or inducing the FBI chief to back off Flynn, an allegation news reports say was based on a memo Comey drafted after meeting with Trump, amounts to concealing an underlying crime, then this could be a misprision case.

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But what about Trump's special status as head of the executive branch—the branch that enforces the law, and commands the Department of Justice? Most misprision cases involve Joe Citizen, who otherwise has no obligation to report crimes. If Trump has a duty to enforce the law, and fails to report a supposed felony by Flynn, isn't that silence alone "concealment," even if he doesn't order Comey to "let it go"?

The courts have grappled with this issue—but mostly in the context of police officers, not the Commander in Chief. There is historical support in common law misprision doctrine, dating back to 1628, suggesting that public officials have special responsibilities. That doctrine considered that the "concealment of felonies in sheriffs, or bailiffs of liberties is more severely punished than in others."

What about in presidents?

It's not a stretch to say that government officials, police and presidents alike, who are already under an affirmative duty to report crimes, inherently conceal when they do not meet their duty to disclose—by simply doing nothing.

And that's just misprision of a felony, a crime few have even heard of. There are arguably plenty of other statutes that might come into play during investigations of Trump's behavior. Federal criminal statutes are written to cover a lot of conduct, with plenty of catchall provisions and lengthy subsections.

That's one reason the US attorneys have well over a 90% conviction rate in criminal cases. Sure, part of that is because they are good at focusing their resources on a case and investigating the heck out of it until it's airtight. A second reason is that the potential federal sentences are so nasty that defendants have incentives to plead out cases. But the third reason demonstrates why it's not too hard to shoehorn a president's activity into a federal crime: federal criminal law casts a very wide net.

But don't forget: Criminal conduct is not required for impeachment.

Section 4 of Article Two of the United States Constitution provides that the president "shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors." Paradoxically enough, this was not an express limitation to criminal conduct.

The framers likely understood "high" to be read with the preceding crimes (treason and bribery) as wrongs against the public or of a political character. Political wrongs recognized under English common law included misapplication of funds and abuse of power, and even interfering with another branch of government.

History provides examples of impeachable but non-criminal conduct. In 1804, District Court Judge John Pickering was impeached because he, "in a most profane and indecent manner, [invoked] the name of the Supreme Being, to the evil example of the good citizens of the United States." As bad as that sounds, it's not a crime, but still got him impeached.

Many years later , the House Judiciary Committee voted to recommend President Nixon's impeachment on grounds including abuse of power. Abuse of power itself is not a crime, but it's impeachable. He resigned before it could happen.

Ultimately, non-criminal conduct must be i mpeachable because the Constitution would never have left us without a way to remove a president who is totally derelict in his duty. If the President simply refused to do any work and decided to golf all day or hide in the Lincoln Bedroom, that would not be a crime, but it would certainly rise above mere non-impeachable "maladministration."

Might a president commit federal crimes other than just obstruction of justice? Could be—but if so, it's only because it's pretty easy to commit a federal crime. But ultimately, a federal crime is not required to impeach a president.