  The analysis and combination of data from different gamma-ray instruments
involves the use of collaboration proprietary software and case-by-case
methods. The effort of defining a common data format for high-level data,
namely event lists and instrument response functions (IRFs), has recently
started for very-high-energy gamma-ray instruments, driven by the upcoming
Cherenkov Telescope Array (CTA). In this work we implemented this prototypical
data format for a small set of MAGIC, VERITAS, FACT, and H.E.S.S. Crab nebula
observations, and we analyzed them with the open-source gammapy software
package. By combining data from $Fermi$-LAT, and from four of the currently
operating imaging atmospheric Cherenkov telescopes, we produced a joint maximum
likelihood fit of the Crab nebula spectrum. Aspects of the statistical errors
and the evaluation of systematic uncertainty are also commented upon, along
with the release format of spectral measurements. The results presented in this
work are obtained using open-access on-line assets that allow for a long-term
reproducibility of the results.
