Create FAQ.md

FAQ.md

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+# FAQs
+
+#### For some given code, changing the value in a constant variable changes the number of constraints in the generated circuit. Is this behavior correct?
+
+**Yes**, take the integers as an example. In Leo, integers are represented as its binary decomposition,
+with each bit occupying one field element (that takes on 0 or 1). Then, for an expression such as `a == 4u32`, the operation to evaluate equality
+would comprise a linear pass of bitwise `AND` operations, comparing every bit in the **variable** value with each bit in the **constant** value.
+
+As the constant value is already known to the compiler during circuit synthesis, the compiler is already able to complete part of the equality evaluation,
+by assuming that any bit in the constant value that is `0` will clearly evaluate to `0`. As such, depending on the value of the constant integer in your code,
+the total number of constraints in the generate circuit can vary.
+
+To illustrate this, here are two examples to show the difference:
+```
+const    = 00000001
+variable = abcdefgh
+---------------------------------
+output   = 0000000h (1 constraint)
+```
+
+```
+const    = 01110001
+variable = abcdefgh
+---------------------------------
+output   = 0bcd000h (4 constraints)
+```