# Reusing output of QEPCAD

Suppose we get a quantifier-free formula as an output from QEPCAD and we want to use that formula as an input for another call to QEPCAD. How can this be done? The naive approach does not work as the following example shows (a very trivial example as the first `qepcad`

call does not actually do much):

```
sage: var("a,b")
sage: formula = qepcad(a+b==0)
sage: print(formula)
b + a = 0
sage: qepcad_formula.exists(b,formula)
```

yields the error

ValueError: Attempting to quantify variable which does not occur in formula