'Constructor
@param method: the aggregation method
@param base: column name of the (base) field
@param slope: column name of the slope field (for cumulate method)
@param interval: time interval expression for the slope'

def __init__(self, method, base, slope=None, interval=None, label=None):

    if (method not in self.METHODS):
        raise SyntaxError(('Unsupported    aggregation    function:    %s' % method))
    self.method = method
    self.base = base
    self.slope = slope
    self.interval = interval
    self.label = label
    self.resource = None
    self.base_rfield = None
    self.base_column = base
    self.slope_rfield = None
    self.slope_column = slope
