Merge pull request #1239 from SciML/ChrisRackauckas-patch-3

Document in FAQ the handling of ifelse, booleans, and registration

Author: ChrisRackauckas

docs/src/basics/FAQ.md

@@ -21,3 +21,25 @@ lowered array? You can use the internal function `varmap_to_vars`. For example:
 ```julia
 pnew = varmap_to_vars([β=>3.0, c=>10.0, γ=>2.0],parameters(sys))
 ```
+
+## How do I handle `if` statements in my symbolic forms?
+
+For statements that are in the `if then else` form, use `IfElse.ifelse` from the
+[IfElse.jl](https://github.com/SciML/IfElse.jl) package to represent the code in a
+functional form. For handling direct `if` statements, you can use equivalent boolean
+mathematical expressions. For example `if x > 0 ...` can be implementated as just
+`(x > 0) * `, where if `x <= 0` then the boolean will evaluate to `0` and thus the
+term will be excluded from the model.
+
+## ERROR: TypeError: non-boolean (Num) used in boolean context?
+
+If you see the error:
+
+```julia
+ERROR: TypeError: non-boolean (Num) used in boolean context
+```
+
+then it's likely you are trying to trace through a function which cannot be
+directly represented in Julia symbols. The techniques to handle this problem,
+such as `@register`, are described in detail 
+[in the Symbolics.jl documentation](https://symbolics.juliasymbolics.org/dev/manual/faq/#Transforming-my-function-to-a-symbolic-equation-has-failed.-What-do-I-do?-1).