The assignment is a unit of the form
where var is a variable of some type and expr an expression of the same type.
The assignment serves to change the value of the variable. It is executed by first executing the expression and then making its value the new value of the variable. The value of all other variables remains unchanged. Thus the assignment
which can be read as ``x becomes x plus one'' does not mean that in some curious way x becomes equal to itself plus one, but rather that first the sum of the present value of x and one is computed and then that value is made to be the new value of x.