\title{Add/subtract sum formulae}
  Simple arithmetic modifications of sum formulae.
addMolecules(formula1, formula2, elements = NULL, maxisotopes = 10)
subMolecules(formula1, formula2, elements = NULL, maxisotopes = 10)
  \item{formula1}{Sum formula}
  \item{formula2}{Sum formula}
  \item{elements}{list of allowed chemical elements, defaults to full
    periodic system of elements}   
  \item{maxisotopes}{maximum number of isotopes shown in the resulting

  addMolecules() adds the second argument to the first.
  subMolecules() subtracts the second argument from the first.

  This can be useful to revert
  e.g. adduct/fragment formation found in ESI mass spectrometry,
  or to mimick simple chemical reactions. No chemical checks are

    A list with the elements
      \item{formula}{repeated sum formula}
      \item{mass}{exact monoisotopic mass of molecule}      
      \item{score}{dummy value, always 1.0}      
      \item{isotopes}{a list of isotopes}

# For proton-Adduct of Ethanol:
subMolecules("C2H7O", "H")

\author{Steffen Neumann <sneumann@IPB-Halle.DE>}