Ok, here's a little primer on the perjury trap: What it is, what it isn't, and whether Bob Mueller's seeking an interview with Trump in which Trump might lie is one or not. /1/
First off, the perjury trap is a real thing. It's not simply a concoction of the right-wing fever swamp. When it happens—and it is successfully argued only rarely—it is a form of entrapment that can constitute an affirmative defense against a charge of perjury. /2/
Bennett Gershman, in a law review article on the subject, described the perjury trap as "the deliberate use of the grand jury to secure perjured testimony" and suggested that the hallmarks include the intentional solicitation of false testimony ... /3/
scholarship.law.upenn.edu/cgi/viewconten…
scholarship.law.upenn.edu/cgi/viewconten…
by a high-value target who cannot be prosecuted for underlying activity for one reason or another. /4/
Second, it is emphatically NOT a perjury trap to put someone in front of a grand jury believing that he might not be entirely truthful and with a willingness to hold him criminally accountable for any lies he might tell. The perjury trap, like all entrapments, requires more. /5/
Here's how the U.S. Attorneys manual describes it: /6/ 
Note two things here:
First, as with any claim of entrapment, the claim of a perjury trap is an affirmative defense that thus has to be proven by a defendant.
Second, the perjury trap requires a showing that the perjurious testimony was procured improperly by the government.
First, as with any claim of entrapment, the claim of a perjury trap is an affirmative defense that thus has to be proven by a defendant.
Second, the perjury trap requires a showing that the perjurious testimony was procured improperly by the government.
In other words, if Bob Mueller seeks an interview with Donald Trump for good faith reasons related to establishing his state of mind in an investigation of certain of his actions knowing—as we all know—that Trump lies constantly, that is not even CLOSE to a perjury trap. /8/
That is called conducting an investigation of a subject with a propensity—one shared by many criminals, by the way—to lie to investigators. The reason we have perjury and false statements statutes is to give such people a strong incentive to tell the truth. /9/
As the US Attorneys manual puts it, "when the grand jury is attempting to obtain useful information in furtherance of its investigation, the perjury trap doctrine does not apply." /10/
Put simply, for the perjury trap argument to be more than simple rhetoric, one would have to show that Mueller was acting in bad faith—was somehow not really seeking evidence for an underlying investigation but was merely setting up Trump to lie. /11/
That's all I got. /12/
