Set plage = Insere(MathTerm.Range, "+")

Function InsertFunction(Functions As OMathFunctions, InsertionPoint As Range, FuncType As WdOMathFunctionType) As OMathFunction
    'Add a function in Functions at InsertionPoint and move InsertionPoint to after the function, ready for next
    Dim NewFunction As OMathFunction
    Set NewFunction = Functions.Add(InsertionPoint, FuncType)
    Set InsertionPoint = NewFunction.Range
    InsertionPoint.Collapse (wdCollapseEnd)
    Set InsertFunction = NewFunction
End Function

Sub InsertText(InsertionPoint As Range, MyText As String)
    'Inserts MyText at InsertionPoint and returns moves InsertionPoint to after that text
    'so this is very similar to
    'Equation.Functions.Add(InsertionPoint, wdOMathFunctionNormalText) .... which does not work!!
    InsertionPoint.Text = MyText
    InsertionPoint.Collapse (wdCollapseEnd)
End Sub

Sub ExampleWriteExpression()
    'insertion point should be in equation. Metric + inverse metric inserted
    Dim Equation As OMath
    Dim MathTerm As OMathFunction
    Dim InsertionPoint As Range

    'If Selection.OMaths.Count <> 1 Then ExpanderFatalError ("Cursor must be in an equation.")
    Set Equation = Selection.OMaths(1)
    Set InsertionPoint = Selection.Range
    
    Set MathTerm = InsertFunction(Equation.Functions, InsertionPoint, wdOMathFunctionScrSub)
    MathTerm.ScrSub.E.Range = "g"
    MathTerm.ScrSub.Sub.Range = ChrW(&H3BC) & ChrW(&H3BD)
    
    Call InsertText(InsertionPoint, "+")

    Set MathTerm = InsertFunction(Equation.Functions, InsertionPoint, wdOMathFunctionScrSup)
    MathTerm.ScrSup.E.Range = "g"
    MathTerm.ScrSup.Sup.Range = ChrW(&H3BC) & ChrW(&H3BD)
End Sub

    InsertionPoint.Start = InsertionPoint.Start - 1       'get into the bracket
    InsertionPoint.Collapse (wdCollapseStart)