User:IssaRice/Colon-equals allows directionality
For definitions, I prefer over other notations like , , and . One reason is that has the counterpart that allows one to place the definiendum on the right. This allows one to e.g. write some long expression (during a calculation), then quickly assign it to something temporarily.
Leary & Kristiansen's logic textbook uses in the meta-level language.