Parsers parsing "a* b* a*" and counting the appearance of the characters.
This parser is implemented in three different ways:
- One built with Metaparse
- One implemented using constexpr functions
- One built with Metaparse but parsing a subexpression with a constexpr parser:
  - a* at the beginning is parsed using a Metaparse parser
  - b* is parsed using a constexpr function
  - a* at the end is parsed using a Mataparse parser again

This example demonstrates how Metaparse can be combined with parsers using
constexpr functions.