Krivine's notation for lambda-terms
Here's a quote from this version which can be found on the author's page (emphasis mine):


  The term $(t)u$ should be thought of as “$t$ applied to $u$” ; it will also be denoted by $t u$ if there is no ambiguity ; the term $(\dots (((t)u_1)u_2)\dots)u_k$ will also be written $(t)u_1u_2\dots u_k$ or $tu_1u_2\dots u_k$. Thus, for example, $(t)uv$, $(tu)v$ and $tuv$ denote the same term.


I'd expect the "if there is no ambiguity" condition to also apply to the second notation (which is just the first notation applied $k-1$ times).