[web2py] SQLFORM.grid dinamically update fields

[alex]

I have a SQLFORM.grid.
When the user updates field 'a' in the grid, before submitting the form, 
field 'b' should represent a calculation based on value of 'a'.
For instance b=a+1

db.py
db.define_table('t',
    Field('a', type='integer'),
    Field('b', type='integer')
    )

default.py
def index():
    grid = SQLFORM.smartgrid(db.t)
    return locals()

index.html
{{extend 'layout.html'}}
{{=grid}}

Someone can help? 

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