# Definition versus equality testing¶

In standard mathematical notation, the = symbol can be used in at least two distinct (yet related) contexts:

• To define things, as in, “Let $$x = 3y + 2$$, and consider…” In this example sentence, $$x = 3y + 2$$ defines the variable x as standing for the expression 3y + 2.
• As a relation which can hold, or not, as in, “If $$x = 3y + 2$$, then … but otherwise …”. In this example sentence, $$x$$ and $$y$$ must already be defined, and $$x = 3y + 2$$ is something that is either true or false.

Notice how the exact same expression $$x = 3y + 2$$ is used in both examples, but means two very different things depending on the context—which is defined entirely by the English words surrounding the symbols! Disco does not have the luxury of using English words to figure out what we mean; instead, Disco must use two different symbols. The = symbol is used to express definitions, as in the first example; whereas the == symbol tests whether two things are equal, as in the second example.