How to code circles around text in Mathjax - mathjax

Question

The following image is from an exercise in Chiswell and Hodges Mathematical Logic, page 21:
I would like to display something similar when I post an answer on Mathematics Stack Exchange, but I don't know how to make the circles with the numbers inside nor the forward slashes through the letters in Mathjax. I think I can handle the fractions with "\frac" as well as the rest of it.
I've looked for examples from other posts to copy, but people seem to replace the circle with parenthesis as an alternative.
Would someone be able to show me how or what software I should use to get this effect?

Answer 1

There are a number of possible ways to get a circle around the one. Here are three:
Use the enclose extension with the circle notation:
\require{enclose}
{\scriptstyle \enclose{circle}{\kern .06em 1\kern .06em}}
You only need to include the \require{enclose} once on the page before your first usage of \enclose. The \kern commands are to make the circle be round rather than oval, since the size of the one is taller than it is wide.
Use \rlap to place the 1 over top of a \bigcirc:
{scriptstyle \rlap{\kern .18em 1}\raise.04em{\bigcirc}}}
we also raise the big circle slightly to get the placement better.
Use the \unicode{} command to obtain the U+2460 character, which is "circled digit one":
{\textstyle\unicode{x2460}}
You can use the cancel package to get \cancel, which produces an diagonal upward slash over its argument, and use a superscript to put the circled 1 in place:
{scriptstyle\cancel{\psi}^{\raise.2em{\enclose{circle}{\kern .06em 1\kern .06em}}}}
Putting this all together with your fractions, you can do something like the following horrible hack:
\require{enclose}\require{cancel}
\frac{
\lower.3em{\scriptstyle\cancel{\psi}\raise.6em{\enclose{circle}{\kern .06em 1\kern .06em}}}
\kern 2.6em
\frac{(\psi\wedge\phi)}{\phi}
{\scriptstyle (\wedge\rm E)}
\kern -2.5em
}{
\kern -.7em
{\scriptstyle\enclose{circle}{\kern .06em 1\kern .06em}}
\frac{(\psi\wedge\phi)}{(\psi\rightarrow(\psi\wedge\phi))}
{\scriptstyle(\rightarrow\rm I)}
\kern -1.7em
}\ {\scriptstyle (\wedge\rm I).}
This includes a lot of spacing adjustments by hand to get the fraction lines to be the right length. It would be painful to do this for very many of these.
Here is the result: