un­der­tilde-gen – A sim­ple macro for plac­ing a tilde un­der a char­ac­ter

The code cre­ates a \math­ord at stan­dard size (no at­tempt to ad­just for dif­fer­ent con­texts in a math­e­mat­i­cal ex­pres­sion).
Ben­jamin Ba­yart's un­der­tilde (for all its faults) does the job bet­ter—the gen­eral user could eas­ily re­move the LaTeX mech­a­nisms from the file.