NEBULAR is a lightweight code to synthesize the spectrum of a
mixed hydrogen helium gas in collisional ionization equilibrium. 
It is not a spectral fitting code, but it can be used to resample 
a model spectrum onto the wavelength grid of a real observation.

Supported are a wide range of temperatures and densities.
NEBULAR includes free-free, free-bound, two-photon and line emission
from HI, HeI and HeII. The code will either return the composite
model spectrum, or - if desired - the unrescaled atomic emission 
coefficients.

WARNING:
Ionization fractions are calculated by solving the Saha equation.
These fractions might be far off the real values in a true astrophysical
nebula; the fractions can be overridden on the command-line using
user-supplied values.
NEBULAR is not a photo-ionization code; if you want that, you must use 
something much more sophisticated such as CLOUDY.

EXAMPLE runs can be found in the appendix of 
http://arxiv.org/abs/1608.03132
This paper also serves as the primary documentation.

To get a list of command-line options, run the nebular executable 
without arguments.