Add an example that returns a sum of values.

[Pegase745]

Fixes #93

[codecov-io]

Codecov Report

Merging #94 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #94   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files           1        1           
  Lines         190      190           
=======================================
  Hits          178      178           
  Misses         12       12

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[Pegase745]

I'll keep it open until I think of a better way