Look no equations.
Simple variable: {x}.
This is an equation: {=1+1}, as is this: {={x}+{y}}.
Here is a more complex one: {=sin(2*pi()*{theta})}.
This is an equation: {=eval(1)}.
Good: {=1+1}, bad: {=eval(1)}, good: {={x}+{y}}.
Bad: {=eval(1)}, bad: {=system(1)}.