def eval_species(start: int, end: int):
mol_start = start
while not toks[mol_start].isalpha() and mol_start < end:
mol_start += 1
if mol_start == end:
raise ValueError('Invalid equation')
coeff = ''.join(toks[start:mol_start])
mol_toks = toks[mol_start:end]
mol = Molecule.complete_formula(''.join(mol_toks))
return Species(mol, coeff=coeff)
