I have a gas model where I want to compare two cases:

- using a fixed “compressibility factor” z
- computing z from an EoS [Equation of State]

The idea is to compare the dynamic behavior. So my idea is to add a `structural_parameters`

block:

```
@structural_parameters begin
z_choice = :fixed
end
```

thus with `z_choice`

as `:fixed`

by default. If I instead want to use a cubic EoS, set `z_choice=:cubic`

.

I have a long list of equations…

```
@equations begin
... ~ ...
z ~ ...
... ~ ...
end
```

**Question**: How do I switch between how I compute `z`

in the *equations*? Is the correct way to do:

```
if z_choice == :fixed
@equations begin
...~...
z ~ 0.9
... ~ ...
end
elseif z_choice == :cubic
@equations begin
... ~ ...
z ~ cubic(...)
... ~ ...
end
end
```

[do I need to enclose the above in a `begin...end`

block?]

**or** can I do the following (saving dramatically on the number of code lines):

```
@equations begin
... ~ ...
if z_choice == :fixed
z ~ 1.3
elseif z_choice == :cubic
z ~ cubic(...)
end
... ~ ...
end
```